Let a crop be trimmed from one edge

Four bar handles at the midpoints of the sides, each moving only its
own side along the axis across it. The overlay reports an edge as 0.5
on the axis it does not move, so the anchor Rust takes is the middle
of the far side.

Under a ratio lock the dragged axis leads. with_aspect grew the short
axis onto the ratio, which for an edge pulled inward made the untouched
axis the leader and pushed the edge straight back out.

ui/dr-ui/src/develop_ui.rs

@@ -1042,9 +1042,10 @@ fn wire_zoom_pan_crop(
                     width,
                     height,
                 };
-                // The overlay reports the corner it is *holding*; the point
-                // that must not move is the opposite one. A move reports no
-                // corner at all, and keeps the shape it already has — there is
+                // The overlay reports the handle it is *holding*; the point
+                // that must not move is the opposite one — the far corner, or
+                // for an edge the middle of the far side. A move reports no
+                // handle at all, and keeps the shape it already has — there is
                 // nothing to reshape, and reshaping about a centre would drag
                 // an over-moved rect smaller instead of sliding it along the
                 // edge.