Fix OIT shaders error with DX12 backend (#15782)

# Objective

- Fixes #15781

## Solution

- DX12 backend seems to require functions with return types to return
value. [WebGPU spec also requires
this](https://gpuweb.github.io/gpuweb/wgsl/#behaviors-rules).

Upstream issue: gfx-rs/wgpu#4458
gpuweb/gpuweb#2523

## Testing

- Tested `order_independent_transparency` example with both dx12 and
vulkan backend on Windows

crates/bevy_core_pipeline/src/oit/oit_draw.wgsl

@@ -4,7 +4,7 @@
 
 #ifdef OIT_ENABLED
 // Add the fragment to the oit buffer
-fn oit_draw(position: vec4f, color: vec4f) -> vec4f {
+fn oit_draw(position: vec4f, color: vec4f) {
  // get the index of the current fragment relative to the screen size
  let screen_index = i32(floor(position.x) + floor(position.y) * view.viewport.z);
  // get the size of the buffer.
@@ -19,15 +19,14 @@ fn oit_draw(position: vec4f, color: vec4f) -> vec4f {
  // accidentally increase the index above the maximum value
  atomicStore(&oit_layer_ids[screen_index], oit_layers_count);
  // TODO for tail blending we should return the color here
- discard;
+ return;
  }
 
  // get the layer_index from the screen
  let layer_index = screen_index + layer_id * buffer_size;
  let rgb9e5_color = bevy_pbr::rgb9e5::vec3_to_rgb9e5_(color.rgb);
  let depth_alpha = pack_24bit_depth_8bit_alpha(position.z, color.a);
  oit_layers[layer_index] = vec2(rgb9e5_color, depth_alpha);
- discard;
 }
 #endif // OIT_ENABLED
 

crates/bevy_core_pipeline/src/oit/resolve/oit_resolve.wgsl

@@ -27,7 +27,12 @@ fn fragment(in: FullscreenVertexOutput) -> @location(0) vec4<f32> {
  let counter = atomicLoad(&layer_ids[screen_index]);
  if counter == 0 {
  reset_indices(screen_index);
- discard;
+
+ // https://github.com/gfx-rs/wgpu/issues/4416
+ if true {
+ discard;
+ }
+ return vec4(0.0);
  } else {
  let result = sort(screen_index, buffer_size);
  reset_indices(screen_index);

crates/bevy_pbr/src/render/pbr.wgsl

@@ -73,8 +73,9 @@ fn fragment(
 #ifdef OIT_ENABLED
  let alpha_mode = pbr_input.material.flags & pbr_types::STANDARD_MATERIAL_FLAGS_ALPHA_MODE_RESERVED_BITS;
  if alpha_mode != pbr_types::STANDARD_MATERIAL_FLAGS_ALPHA_MODE_OPAQUE {
- // This will always return 0.0. The fragments will only be drawn during the oit resolve pass.
- out.color = oit_draw(in.position, out.color);
+ // The fragments will only be drawn during the oit resolve pass.
+ oit_draw(in.position, out.color);
+ discard;
  }
 #endif // OIT_ENABLED