Give every mask an eye and a colour, and put the brush where the mask is

The first build of seeing a mask showed the selected layer's, in one global
style, from a strip at the top of the panel. It answered the wrong question and
answered it somewhere nobody looked. What a photographer asks of two masks is
how they meet — where the sky's edge sits against the building's — and that
needs both on screen at once, in colours that can be told apart.

So each row of the stack has an eye, drawn in the colour its mask is shown in,
and each mask has six swatches to choose that colour from. Several can be open
at once; a new one comes up open, in the first colour nothing else is using.
The style — tint, alpha, outline — is the one setting that stays global, above
the stack, because three styles at once are three pictures that cannot be read
against each other. Alpha now draws every shown mask, each in its colour, on
black. In the pipeline a `Reveal` is a list of `(layer, colour)` rather than
one layer, and every reveal block carries its own colour.

The brush moves too. Select, Paint and Erase and the three sliders under them
sat at the top of the panel, appeared only once a row was selected, and said
nothing about which mask they acted on — so "how do I paint" and "how do I
correct the model's outline" both had the same answer and nobody found it.
They sit under the selected mask's parts now, beside the swatches, and on a
subject or a category the hint says what a stroke there does: it becomes a
part of this mask, joined to the model's, and can be taken out again.

Eyes and colours are viewing state, on the session and not on the layer, so a
photograph reopened has every eye closed — the stored-mask round-trip test
asserts it.

docs/gestures.md

@@ -5,7 +5,7 @@
 
 Every entry here is extracted from the comment beside the code that implements it, so this file cannot describe a gesture the application does not have. Add one by writing a `GESTURE:` block next to the implementation; there is nowhere else to write it.
 
-36 gestures, in 4 places.
+37 gestures, in 4 places.
 
 ## Develop
 
@@ -139,7 +139,16 @@ The column is 280px wide and the colour mixer alone puts thirty-six of these in
 
 Disabling a layer is the before-and-after a local edit constantly wants, so it is one press away rather than inside the row. It is an edit and does take a history step, unlike holding "Before" — the layer really is off until it is switched back on.
 
-<sub>`ui/dr-ui/ui/masks.slint:196`</sub>
+<sub>`ui/dr-ui/ui/masks.slint:209`</sub>
+
+### Show or hide one mask on the photograph
+
+- **Touch** — Tap the eye on its row
+- **Pointer** — Click the eye on its row
+
+A mask is judged by seeing where it falls, and two are judged by seeing where they meet — so each row has its own eye rather than the panel having one, and the eye is drawn in the colour the mask shows in, so the row says which shape on the picture is its. Nothing about the edit changes: this is how the photograph is looked at, and takes no history step.
+
+<sub>`ui/dr-ui/ui/masks.slint:274`</sub>
 
 ## Collections sidebar