Adds linear gradient support to 2D meshes in the canvas widget.

wgpu/src/shader/triangle_solid.wgsl

@@ -0,0 +1,18 @@
+// uniforms
+struct SolidUniforms {
+    transform: mat4x4<f32>,
+    color: vec4<f32>
+}
+
+@group(0) @binding(0)
+var<uniform> solid_uniforms: SolidUniforms;
+
+@vertex
+fn vs_main(@location(0) input: vec2<f32>) -> @builtin(position) vec4<f32> {
+    return solid_uniforms.transform * vec4<f32>(input.xy, 0.0, 1.0);
+}
+
+@fragment
+fn fs_solid() -> @location(0) vec4<f32> {
+    return solid_uniforms.color;
+}