Vulkan Backend Not Doing Gamma Correction/sRGB Conversion

[zicklag]

This was first reported in #1627, initially misinterpreted as a bug with the GL backend not being colored correctly

Description
Vulkan backend is not doing sRGB conversion or gamma correction resulting in a darker-than-normal image:

Repro steps
Run any of the examples such as cube or shadow using the Vulkan backend. Reproduced so far on:

• Linux Nvidia Vulkan
• Linux Intel Vulkan
• Windows Intel Vulkan

Expected vs observed behavior

Expected:

Observed:

Extra materials
None applicable so far.

Platform
WGPU @ 4b4e393 on Linux with:

• Intel(R) UHD Graphics (CML GT2)
• Using GeForce GTX 1650

[kvark]

I checked Vulkan code, and it seems alright. Wgpu-core is asking for SRGB first, and Vulkan backend is expected to allow that. Would you be able to step through fn surface_capabilities on Vulkan adapter to see why SRGB isn't getting through?

[zicklag]

Adding debug prints shows that the Bgra8UnformSrgb format is selected as the only available format:

[wgpu-hal/src/vulkan/adapter.rs:1057] format = Rgba8Unorm
[wgpu-hal/src/vulkan/adapter.rs:1063] supported = false
[wgpu-hal/src/vulkan/adapter.rs:1057] format = Rgba8UnormSrgb
[wgpu-hal/src/vulkan/adapter.rs:1063] supported = false
[wgpu-hal/src/vulkan/adapter.rs:1057] format = Bgra8Unorm
[wgpu-hal/src/vulkan/adapter.rs:1063] supported = false
[wgpu-hal/src/vulkan/adapter.rs:1057] format = Bgra8UnormSrgb
[wgpu-hal/src/vulkan/adapter.rs:1063] supported = true
[wgpu-hal/src/vulkan/adapter.rs:1068] formats = [
    Bgra8UnormSrgb,
]

But... I took a quick look at PrivateCapabilities::map_texture_format and I found that the Bgra8UnormSrgb was mapped to vulkan's B8G8R8_UNORM, not B8G8R8_SRGB! Fixing that works!

Thanks for telling me where to look @kvark !

I'll have a PR in a minute.

Edit: #1651