Do not divide by one to remap a box that spans its parent

A retained part is re-expressed as a fraction of its new box by dividing by
the old box's extent, and that extent is one whenever the box spans the whole
of its parent's -- which is the common shape. An integer division is the most
expensive thing in `apply_scalar` and it ran twice per span.

`perf stat -e instructions:u` over 500 frames: `many` 1,938,264,572 to
1,886,265,821, `scroll` 452,517,906 to 444,792,314.

Tried first and reverted: short-circuiting a fraction of nought or one, at
either end of the box. That is not the common case, and the two comparisons
cost 17% more than the divisions they were meant to save.

Checked: fmt, clippy, 105 tests, three shrinker cases at 300 seeds, 100
generated seeds.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

core/src/ui/render_state.rs

@@ -1090,7 +1090,13 @@ impl RegionRemap {
         if from.len() == to.len() || extent == Rel::ZERO {
             return scalar + to.start - from.start;
         }
-        let fraction = (scalar.rel - from.start.rel) / extent;
+        // A box that spans the whole of its parent's is the common one, and
+        // dividing by one is the expensive way to write a subtraction.
+        let offset = scalar.rel - from.start.rel;
+        let fraction = match extent == Rel::ONE {
+            true => offset,
+            false => offset / extent,
+        };
         let from_px = fraction.lerp(from.start.px, from.end.px);
         let to_rel = fraction.lerp(to.start.rel, to.end.rel);
         let to_px = fraction.lerp(to.start.px, to.end.px);