perf: keep heavy work off UI thread

crates/iced_wgpu/src/shader/triangle/solid.wgsl

@@ -0,0 +1,24 @@
+struct SolidVertexInput {
+    @location(0) position: vec2<f32>,
+    @location(1) color: vec4<f32>,
+}
+
+struct SolidVertexOutput {
+    @builtin(position) position: vec4<f32>,
+    @location(0) color: vec4<f32>,
+}
+
+@vertex
+fn solid_vs_main(input: SolidVertexInput) -> SolidVertexOutput {
+    var out: SolidVertexOutput;
+
+    out.color = premultiply(input.color);
+    out.position = globals.transform * vec4<f32>(input.position, 0.0, 1.0);
+
+    return out;
+}
+
+@fragment
+fn solid_fs_main(input: SolidVertexOutput) -> @location(0) vec4<f32> {
+    return input.color;
+}