DXC Integration in DX12 Backend

[cwfitzgerald]

Is your feature request related to a problem? Please describe.

We currently use fxc (aka d3dcompiler) to compile our HLSL to DXBC. It has a couple issues:

• Completely unmaintained, with many issues
• Extremely slow
• DXBC is limited to SM5.1 which limits the features we can offer.
  • Subgroup Ops
  • Native float16

While our end goal is to have naga have a DXIL backend, that is a lot of work and a long way away, so an intermediate step is to integrate with DXC.

Because DXC is a runtime dependency, we still need to be able to fall back to FXC if DXC isn't available, and to get these features users will need to ship dxcompiler.dll with their application.

Describe the solution you'd like

Integration with DXC, possibly through the hassle-rs crate.

Describe alternatives you've considered

Waiting for a DXIL backend for naga. No C code but will take a long while.

Additional context

See this naga issue for some of the problems we've needed to work around to get FXC to work gfx-rs/naga#1918.