Rename a length's abs component to px

`dp` is coming, and then `abs` says which of the two it is not. The
component has always been a pixel count, so the name that admits it is
the one that leaves room for a second unit beside it.

Mechanical: the field on `Len` and `UiScalar`, their constructors,
`to_abs`/`get_abs`, the matching WGSL struct member and the locals
composing it. Field order and types are unchanged, so the `Pod` layout
the shader reads is the same bytes. `f32::abs` is untouched.

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

core/src/render/shader/prelude.wgsl

@@ -40,7 +40,7 @@ const CHAIN_LIMIT: u32 = 64u;
 fn scalar_within(s: UiScalar, p: UiSpan) -> UiScalar {
     return UiScalar(
         mix(p.start.rel, p.end.rel, s.rel),
-        s.abs + mix(p.start.abs, p.end.abs, s.rel),
+        s.px + mix(p.start.px, p.end.px, s.rel),
     );
 }
 
@@ -69,7 +69,7 @@ struct UiSpan {
 
 struct UiScalar {
     rel: f32,
-    abs: f32,
+    px: f32,
 }
 
 struct InstanceInput {
@@ -104,12 +104,12 @@ fn vs_main(
     );
     let r = resolve_move(in.move_idx, local);
     let top_left_rel = vec2(r.x.start.rel, r.y.start.rel);
-    let top_left_abs = vec2(r.x.start.abs, r.y.start.abs);
+    let top_left_px = vec2(r.x.start.px, r.y.start.px);
     let bot_right_rel = vec2(r.x.end.rel, r.y.end.rel);
-    let bot_right_abs = vec2(r.x.end.abs, r.y.end.abs);
+    let bot_right_px = vec2(r.x.end.px, r.y.end.px);
 
-    let top_left = floor(top_left_rel * window.dim) + floor(top_left_abs);
-    let bot_right = floor(bot_right_rel * window.dim) + floor(bot_right_abs);
+    let top_left = floor(top_left_rel * window.dim) + floor(top_left_px);
+    let bot_right = floor(bot_right_rel * window.dim) + floor(bot_right_px);
     let size = bot_right - top_left;
 
     let uv = vec2<f32>(
@@ -136,12 +136,12 @@ fn masked(in: VertexOutput, color: vec4<f32>) -> vec4<f32> {
     // clips content that moves inside it.
     let m = resolve_move(mask.move_idx, Region(mask.x, mask.y));
     let tl = vec2(m.x.start.rel, m.y.start.rel);
-    let tl_abs = vec2(m.x.start.abs, m.y.start.abs);
+    let tl_px = vec2(m.x.start.px, m.y.start.px);
     let br = vec2(m.x.end.rel, m.y.end.rel);
-    let br_abs = vec2(m.x.end.abs, m.y.end.abs);
+    let br_px = vec2(m.x.end.px, m.y.end.px);
 
-    let top_left = floor(tl * window.dim) + floor(tl_abs);
-    let bot_right = floor(br * window.dim) + floor(br_abs);
+    let top_left = floor(tl * window.dim) + floor(tl_px);
+    let bot_right = floor(br * window.dim) + floor(br_px);
     let pos = in.clip_position.xy;
     if pos.x < top_left.x || pos.x > bot_right.x || pos.y < top_left.y || pos.y > bot_right.y {
         return color * 0.0;

core/src/render/shader/rect.wgsl

@@ -34,6 +34,6 @@ fn distance_from_rect(pixel_pos: vec2<f32>, rect_center: vec2<f32>, rect_corner:
     // vec from center to pixel
     let p = pixel_pos - rect_center;
     // vec from inner rect corner to pixel
-    let q = abs(p) - (rect_corner - radius);
+    let q = px(p) - (rect_corner - radius);
     return length(max(q, vec2(0.0))) - radius;
 }