Show the file's own pixels at 1:1 and beyond

Zoomed to 1:1 or past it, the develop canvas showed a smoothed blur
rather than the photograph's pixels, so focus and noise could not be
judged at the magnification meant for judging them.

Two things caused it. The canvas only switched to nearest-neighbour
strictly past 1:1, with a margin, so the 1:1 inspection itself stayed
smooth. And the switch mostly had nothing to act on: the pipeline
rendered a viewport-sized frame at every zoom, so past 1:1 it was the
pipeline doing the enlarging - bilinearly whenever a straightening angle
or lens correction was in the chain - and the detail stage then sharpened
and denoised those invented pixels at radii scaled up to match. The
texture reached the canvas already blurred and was presented 1:1.

Now, from 1:1 on, the visible region is rendered at the source's own
resolution (render::render_size) and the canvas enlarges it with
nearest-neighbour, so the blocks on screen are the pixels an export would
have; it is also less shading. The decision lives in two small
functions, render::magnification and render::shows_source_pixels,
measured in physical pixels like one_to_one_zoom, with a half-percent
tolerance so the inspection zoom counts as 1:1 even where fit() rounded
the other edge. Below 1:1 the render and the smooth filter are unchanged.

docs/gestures.md

@@ -23,9 +23,9 @@ Sampling a neutral is the first move of the tonal pass — every colour judgemen
 - **Touch** — Pinch it with two fingers
 - **Pointer** — The scroll wheel over it
 
-Anchored on the fingers' midpoint, and on the pointer, so the gesture reads as magnifying the picture rather than sliding it about. Double-tap is the way to an exact 1:1; this is the way to everything in between.
+Anchored on the fingers' midpoint, and on the pointer, so the gesture reads as magnifying the picture rather than sliding it about. Double-tap is the way to an exact 1:1; this is the way to everything in between. Past 1:1 the pixels are shown as they are, square and unsmoothed; below it, filtered.
 
-<sub>`ui/dr-ui/ui/app.slint:1707`</sub>
+<sub>`ui/dr-ui/ui/app.slint:1715`</sub>
 
 ### Move a magnified photograph about
 
@@ -34,7 +34,7 @@ Anchored on the fingers' midpoint, and on the pointer, so the gesture reads as m
 
 Only once there is something outside the viewport to reach, which is why the cursor becomes a hand exactly then. The view is clamped to the frame: panning past the edge would show undefined area beside the photograph, and that reads as a rendering fault rather than as the end of the picture.
 
-<sub>`ui/dr-ui/ui/app.slint:1798`</sub>
+<sub>`ui/dr-ui/ui/app.slint:1808`</sub>
 
 ### Paint a mask by hand
 
@@ -43,7 +43,7 @@ Only once there is something outside the viewport to reach, which is why the cur
 
 A model's mask stops inside a shoulder and leaks into the hair, and no single edge control fixes two errors that go opposite ways. The whole stroke is one step in the history, so taking a mark back costs one press however long it took to make.
 
-<sub>`ui/dr-ui/ui/app.slint:1885`</sub>
+<sub>`ui/dr-ui/ui/app.slint:1895`</sub>
 
 ### Take back the last change
 
@@ -53,7 +53,7 @@ A model's mask stops inside a shoulder and leaks into the hair, and no single ed
 
 A whole drag is one step, so undo takes back a decision rather than a frame of a gesture. The list is there because arriving six steps back costs what arriving from one does.
 
-<sub>`ui/dr-ui/ui/app.slint:2113`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2123`</sub>
 
 ### Do it again after taking it back
 
@@ -61,7 +61,7 @@ A whole drag is one step, so undo takes back a decision rather than a frame of a
 - **Pointer** — Click it, or press Redo in the History header
 - **Keyboard** — Ctrl+Shift+Z
 
-<sub>`ui/dr-ui/ui/app.slint:2126`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2136`</sub>
 
 ### Copy the settings from this photograph
 
@@ -71,7 +71,7 @@ A whole drag is one step, so undo takes back a decision rather than a frame of a
 
 The button is the copy that has to work: a tablet has no modifier key to hold and no menu bar to hang the action from. The shortcut is an accelerator for a control that is on screen either way.
 
-<sub>`ui/dr-ui/ui/app.slint:2159`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2169`</sub>
 
 ### Paste the settings onto this photograph
 
@@ -81,7 +81,7 @@ The button is the copy that has to work: a tablet has no modifier key to hold an
 
 The button names what would be pasted — "3 adjustments", and whether the crop is coming with it — which the shortcut cannot say. Both paste the same scope.
 
-<sub>`ui/dr-ui/ui/app.slint:2171`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2181`</sub>
 
 ### Choose which kinds of edit a copy carries
 
@@ -91,7 +91,7 @@ The button names what would be pasted — "3 adjustments", and whether the crop
 
 Lightroom's Copy Settings. Pasting a look across a shoot usually means leaving each frame's crop and rotation alone, and that is a choice to make at the moment of copying.
 
-<sub>`ui/dr-ui/ui/app.slint:2188`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2198`</sub>
 
 ### Export this photograph as the last one was
 
@@ -101,7 +101,7 @@ Lightroom's Copy Settings. Pasting a look across a shoot usually means leaving e
 
 Every export runs on the defaults in Settings, so "as the last one was" is what the button already does. The chord is Lightroom's and darktable's, kept so hands that learned it there need not learn it again.
 
-<sub>`ui/dr-ui/ui/app.slint:2217`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2227`</sub>
 
 ### Choose how to export, then export
 
@@ -111,7 +111,7 @@ Every export runs on the defaults in Settings, so "as the last one was" is what
 
 The export sheet is the export defaults alone with an Export button. What is chosen there is kept, so it is also what the next Ctrl+Shift+E uses.
 
-<sub>`ui/dr-ui/ui/app.slint:2229`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2239`</sub>
 
 ### Change which group of adjustments is on screen
 
@@ -121,7 +121,7 @@ The export sheet is the export defaults alone with an Export button. What is cho
 
 The groups are whatever the operation set declares itself to be about, so there are as many as the pipeline has and no key can be assigned to one of them by name. Stepping is the binding that survives a node being added.
 
-<sub>`ui/dr-ui/ui/app.slint:2254`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2264`</sub>
 
 ### Look at the photograph at 1:1
 
@@ -129,9 +129,9 @@ The groups are whatever the operation set declares itself to be about, so there
 - **Pointer** — Double-click it, or press the zoom readout floating over the canvas
 - **Keyboard** — Z
 
-Noise reduction and capture sharpening are judgements about single pixels, and a fitted view averages several of the file's into each one on screen — so the frame looks softer than it is and the correction goes too far. The point and the magnification survive opening the next photograph, which is what makes checking the same eye across forty portraits forty keystrokes rather than forty pans.
+Noise reduction and capture sharpening are judgements about single pixels, and a fitted view averages several of the file's into each one on screen — so the frame looks softer than it is and the correction goes too far. The point and the magnification survive opening the next photograph, which is what makes checking the same eye across forty portraits forty keystrokes rather than forty pans. From 1:1 on the photograph is drawn as its own pixels, each a hard-edged square, rather than smoothed into a blur.
 
-<sub>`ui/dr-ui/ui/app.slint:2289`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2299`</sub>
 
 ### Move to the next or previous photograph
 
@@ -141,7 +141,7 @@ Noise reduction and capture sharpening are judgements about single pixels, and a
 
 The edit on screen is saved on the way out, so stepping through a folder is as much a departure as going back to the grid and loses nothing. A and D as well as the arrows, so the left hand steps along the roll while the right stays on the mouse. Unmodified only: Ctrl+D and Ctrl+A are not this.
 
-<sub>`ui/dr-ui/ui/app.slint:2341`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2354`</sub>
 
 ### See the photograph before you edited it
 
@@ -151,7 +151,7 @@ The edit on screen is saved on the way out, so stepping through a folder is as m
 
 Held rather than toggled, and no split screen: a split halves the working image on the tablet the column was sized for, and the comparison photographers describe making is a flick back and forth. It takes no history step, so checking whether a frame is overcooked costs nothing to undo afterwards.
 
-<sub>`ui/dr-ui/ui/app.slint:2474`</sub>
+<sub>`ui/dr-ui/ui/app.slint:2487`</sub>
 
 ### Put one control back to its default