Add StorageFormat::Bgra8Unorm

[nical]

Fixes #2195
Blocks: gfx-rs/wgpu#3634

I'm not sure what the best course is for the wgsl backend, so I defaulted to putting the "rgba8" swizzle which is the closest available format. I suppose that the backend would have to insert some extra code to convert when interacting with the storage texture (probably that's already done in other places?).

[nical]

Out of curiosity I looked at tint's solution and they do some substitution between bgra and rgba in their IR, see https://dawn.googlesource.com/tint/+/refs/heads/main/src/tint/lang/core/ir/transform/bgra8unorm_polyfill.cc

[teoxoy]

Out of curiosity I looked at tint's solution and they do some substitution between bgra and rgba in their IR, see https://dawn.googlesource.com/tint/+/refs/heads/main/src/tint/lang/core/ir/transform/bgra8unorm_polyfill.cc

That transform only runs for SPIR-V. The bgra8 -> rgba8 format replacement is not valid, they should use Unknown because SPIR-V's rgba8 format is not compatible with the bgra8 one that we'll use on the API side.

The swizzle I think we probably need but we should probably test first.

I'm not sure what the best course is for the wgsl backend, so I defaulted to putting the "rgba8" swizzle which is the closest available format. I suppose that the backend would have to insert some extra code to convert when interacting with the storage texture (probably that's already done in other places?).

I assume you mean the GLSL backend, we should probably fail with an unimplemented/unsupported error.