[hal/dx12] Mesh Shaders

[SupaMaggie70Incorporated]

Connections
Addresses #7219, a sub-issue of #7197

Description
This adds a DX12 backend for mesh shaders

Current issues:

• Limits aren't calculated properly
• In the pipeline stream descriptor, we could figure out the maximum length and possibly avoid allocations and vec's if that's desirable

Testing
I have updated the mesh shader example to use DXILpassthrough to test this, and it works.

Squash or Rebase?
Squash

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy --tests. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests.
• If this contains user-facing changes, add a CHANGELOG.md entry.

[cwfitzgerald]

Broadly looks good, I have some questions/concerns. I'm definitely going to want to do at least one more pass where I actually clone this locally to poke around.

[cwfitzgerald]

Too may bools here - can we use a enum(s)?

[SupaMaggie70Incorporated]

I've tried to address this by using a struct, not sure its much of an improvement at all though.

[cwfitzgerald]

You should add "Fo" to the list of extend-words or identifiers, not block the whole shader.

[SupaMaggie70Incorporated]

I think that "Fo"->"Do" or "Fo"->"Go" or "Fo"->"To" etc are close enough on the keyboard and common enough that it would be worth more to keep Fo disallowed than to block it for a single function used in 2 files.