Let a zoomed view fill the viewport rather than keep the photograph's shape

The view was the same fraction of each axis, so it kept the frame's
aspect at every zoom: a portrait zoomed on a landscape screen stayed a
portrait strip with the screen's sides empty. Each axis now shows as
much of the frame as the viewport holds at that magnification, capped
at the whole frame, and the render is fitted to the viewed region
rather than to the frame. A redraw re-cuts a zoomed view about its
centre when the viewport or the crop changes shape.

ui/dr-ui/src/develop_ui.rs

@@ -895,10 +895,12 @@ fn wire_zoom_pan_crop(
         let weak = window.as_weak();
         let session = session.clone();
         let redraw = redraw.clone();
+        let viewport = viewport.clone();
         window.on_zoom_at(move |factor, at_x, at_y| {
             let Some(w) = weak.upgrade() else { return };
+            let (vw, vh) = *viewport.borrow();
             if let Some(s) = session.borrow_mut().as_mut() {
-                s.zoom_about(factor, at_x, at_y);
+                s.zoom_about(factor, at_x, at_y, vw, vh);
             }
             redraw(&w);
         });