fix: MSAA resolve no longer overwrites widget backgrounds

MSAA and depth textures are now sized to the shader widget's clip
bounds instead of the full surface. The resolve writes to a matching
clip-sized intermediate texture; a blit pass (blit.wgsl, viewport
trick) then copies the result to the exact clip region of the surface
target, leaving all other widgets untouched.

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

src/shaders/blit.wgsl

@@ -0,0 +1,44 @@
+// blit.wgsl
+// Copies the MSAA-resolved texture to the surface target.
+// A full-screen NDC quad is drawn; the render pass viewport positions it
+// exactly over the shader widget's clip bounds in the surface.
+
+struct VertexOutput {
+    @builtin(position) position: vec4<f32>,
+    @location(0) uv: vec2<f32>,
+}
+
+var<private> POSITIONS: array<vec2<f32>, 6> = array<vec2<f32>, 6>(
+    vec2<f32>(-1.0,  1.0),
+    vec2<f32>(-1.0, -1.0),
+    vec2<f32>( 1.0,  1.0),
+    vec2<f32>(-1.0, -1.0),
+    vec2<f32>( 1.0, -1.0),
+    vec2<f32>( 1.0,  1.0),
+);
+
+// NDC Y is up; texture V is down — flip Y.
+var<private> UVS: array<vec2<f32>, 6> = array<vec2<f32>, 6>(
+    vec2<f32>(0.0, 0.0),
+    vec2<f32>(0.0, 1.0),
+    vec2<f32>(1.0, 0.0),
+    vec2<f32>(0.0, 1.0),
+    vec2<f32>(1.0, 1.0),
+    vec2<f32>(1.0, 0.0),
+);
+
+@vertex
+fn vs_main(@builtin(vertex_index) idx: u32) -> VertexOutput {
+    var out: VertexOutput;
+    out.position = vec4<f32>(POSITIONS[idx], 0.0, 1.0);
+    out.uv = UVS[idx];
+    return out;
+}
+
+@group(0) @binding(0) var t_resolved: texture_2d<f32>;
+@group(0) @binding(1) var s_resolved: sampler;
+
+@fragment
+fn fs_main(in: VertexOutput) -> @location(0) vec4<f32> {
+    return textureSample(t_resolved, s_resolved, in.uv);
+}