Mesh shaders

[Firestar99]

Depends on #43

Diff without dependent PR: Firestar99/rust-gpu-new@update-rspirv...Firestar99:rust-gpu-new:mesh-shaders

changelog:

- [PR#44](https://github.com/Rust-GPU/rust-gpu/pull/44) added support for mesh and task shaders

[LegNeato]

It would be good to have a test, changelog entry, and an example for this. Might be good to do a blog post too!

[Firestar99]

Plenty of tests already exist in this PR and I added a changelog entry in the description. I can make you a quick example shader later.

[Firestar99]

There are two vendor-specific issues I know of with regards to "payloads" (sending data from task to mesh shader):

• AMD on Windows assumes the payload is always a struct, even if the spec says it could be anything like an uint. It not being a struct will segfault the driver on pipeline creation. If you define a struct with just a single primitive member, like a uint, rust-gpu will "inline" the struct and make the payload be a uint. Workaround: add a second dummy member.
• RenderDoc suffers from the same issue as above, and will display something like "shader likely compiled by some old weird dxc version"
• RenderDoc assumes the struct for the payload is never constructed, only it's fields written to. For it's debugging, it will inject a new field into said struct to emit some debug info. If you just assign fields to that struct it's fine, but it won't fixup the OpConstruct statements to make a new struct, resulting in invalid spirv and RenderDoc complaining. Rust-gpu merges structs with the same members, making it rather easy to trip onto this one.

Suggested fix: Wrap all task shader payloads in a separate struct that is not optimized away.

[LegNeato]

Plenty of tests already exist in this PR and I added a changelog entry in the description. I can make you a quick example shader later.

Oh weird, they didn't show up on mobile!

[LegNeato]

@eddyb or @schell , I don't know anything about mesh shaders...can either of you review?

[Firestar99]

I think nobody but me knows about mesh shaders...
I wouldn't worry too much about correctness of the mesh shader stuff, I've tested it plenty so hopefully all issues have already been caught.

[LegNeato]

Let's land this and iterate if needed, it has been sitting too long.

[LegNeato]