dx12: Enable Rgba8Unorm storage textures for DirectComposition transparency

[joeleaver]

Summary

This PR forces storage texture support for Rgba8Unorm, Rgba8Snorm, and Bgra8Unorm formats on DX12. These formats are widely supported for UAV typed store on modern GPUs, but DX12's D3D12_FORMAT_SUPPORT2_UAV_TYPED_STORE query may not report this correctly.

This is needed for GPU-accelerated 2D rendering libraries like Vello to work with transparent windows using DirectComposition on Windows.

Changes

Single change in instance.rs: Force STORAGE_BINDING support for the three common 8-bit RGBA formats after the standard capability detection.

Background

When using Vello with DirectComposition for transparent windows on Windows:

1. DX12 backend is required (for WGPU_DX12_PRESENTATION_SYSTEM=DxgiFromVisual)
2. Vello's compute shaders need STORAGE_BINDING on Rgba8Unorm textures
3. Modern GPUs support this, but the DX12 format query may not report it correctly

The error without this patch:

The adapter does not support write access for storage textures of format Rgba8Unorm

Testing

• Tested on Windows 11 with NVIDIA RTX 3070 Ti
• Vello successfully renders to transparent DirectComposition windows
• No validation errors from wgpu

Revision Notes

Based on @DJMcNab's feedback, I've simplified this PR to only include the instance.rs change. The other changes (to device/resource.rs and present.rs) were not necessary since Vello renders to an intermediate texture, not directly to the swapchain.

🤖 Generated with Claude Code

[inner-daemons]

Thanks for the AI disclaimer at the top lmao, very much appreciated

[DJMcNab]

So to clarify, are you unable to get Vello's with_winit example to run on your device (with the given environment variable value)? I can see how the change in instance.rs could plausibly fix that, if, as you claim, that the underlying adapter misreports that you cannot write to any storage textures with that backend.

I'm less convinced that the other changes should have an impact here, given that you're not trying to use Vello to directly target a render target (as Vello's docs tell you not to, and indeed a scan of your code confirms isn't happening).

[joeleaver]

@DJMcNab You were right! I tested with only the instance.rs change and transparent windows work correctly. I've simplified this PR to remove the other changes (device/resource.rs and present.rs) since they weren't needed.

To answer your question: yes, I can confirm Vello's with_winit example runs on my device. The issue was specifically with transparent windows via DirectComposition, where Vello renders to an intermediate texture (as you noted from the docs). The instance.rs change enables the storage texture capability that Vello's compute shaders need.

Thanks for the review guidance!