Align corner clipping, antialiasing, and fractional material regions

compositor/src/render/shaders/corners.glsl

@@ -6,5 +6,6 @@ float rounded_rect_distance(vec2 point, vec4 rect, float radius) {
 }
 
 float edge_coverage(float distance) {
-    return 1.0 - smoothstep(-0.5, 0.5, distance);
+    // Match Iced/WGPU quad edges: a one-physical-pixel linear ramp.
+    return clamp(0.5 - distance, 0.0, 1.0);
 }

compositor/src/render/shaders/rounded_solid_shader.frag

@@ -0,0 +1,13 @@
+precision highp float;
+uniform float alpha;
+uniform vec4 clip_rect;
+uniform float radius;
+uniform vec4 color;
+
+//_CORNERS_
+
+void main() {
+    float distance = rounded_rect_distance(gl_FragCoord.xy, clip_rect, radius);
+    // Single-pixel buffer colors are already premultiplied.
+    gl_FragColor = color * alpha * edge_coverage(distance);
+}