Added support for custom shader widget for iced_wgpu backend.

examples/custom_shader/src/shaders/depth.wgsl

@@ -0,0 +1,48 @@
+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)
+);
+
+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, 1.0),
+    vec2<f32>(0.0, 0.0),
+    vec2<f32>(1.0, 0.0),
+    vec2<f32>(1.0, 1.0)
+);
+
+@group(0) @binding(0) var depth_sampler: sampler;
+@group(0) @binding(1) var depth_texture: texture_2d<f32>;
+
+struct Output {
+    @builtin(position) position: vec4<f32>,
+    @location(0) uv: vec2<f32>,
+}
+
+@vertex
+fn vs_main(@builtin(vertex_index) v_index: u32) -> Output {
+    var out: Output;
+
+    out.position = vec4<f32>(positions[v_index], 0.0, 1.0);
+    out.uv = uvs[v_index];
+
+    return out;
+}
+
+@fragment
+fn fs_main(input: Output) -> @location(0) vec4<f32> {
+    let depth = textureSample(depth_texture, depth_sampler, input.uv).r;
+
+    if (depth > .9999) {
+        discard;
+    }
+
+    let c = 1.0 - depth;
+
+    return vec4<f32>(c, c, c, 1.0);
+}