Support 16-bit unorm/snorm formats

[fintelia]

This PR adds handling for 16-bit unorm and snorm formats.

After making a few tweaks to wgpu, I was able to use this patch to load and run several GLSL compute shaders operating on r16 unorm textures. I haven't tested the other frontends/backends but the code I added mirrors the existing cases, so there's a good chance they should behave properly

[jimblandy]

This looks interesting, but before I review in detail:

We can't unconditionally extend WGSL, since that affects Firefox's implementation of the WebGPU standard, and we'd introduce incompatibilities between browsers. This probably needs to be conditional on some new flag in naga::valid::Capabilities.

[fintelia]

Looks like #2162 lays out what the capability should be. I'll see about adding suitable validation checks when I have a chance

[teoxoy]

LGTM, but could you also add them to the SPV front-end?

[jimblandy]

The WGSL changes need to be conditional, as noted in the conversation.

[jimblandy]

@teoxoy points out that #2161 covers making the non-WebGPU formats we already support in the WGSL front end conditional, and that's in milestone 4. So I think this is fine to merge as it is.

[teoxoy]

Sorry, I approved it without reading the comments properly.

I thought gfx-rs/wgpu#4412 would cover this and could be done in a separate PR since there are a bunch of other formats. But ideally this PR should close #2162 (we don't yet have the framework for naga specific extensions but we can move that point from the issue into gfx-rs/wgpu#4410).

@fintelia adding the Capability would be appreciated since we have to wire it to wgpu as well.

[teoxoy]

Thanks for introducing the new capability!
Could you also add the new formats to the spv front-end?