Updated Dxc integration for DX12 backend

[Elabajaba]

Checklist

• Run cargo clippy.
• Run RUSTFLAGS=--cfg=web_sys_unstable_apis cargo clippy --target wasm32-unknown-unknown if applicable.
• Add change to CHANGELOG.md. See simple instructions inside file.
• Figure out how we should enable and disable DXC, and whether that's at runtime/compiletime/startup/etc
• Figure out what the fallback should be if the user enables DXC, but the .dlls aren't available (crash with an error message? Fall back to fxc but log the error)
• Decide if we're going to allow users to set an explicit path to the .dll location
• Figure out why it's working on my machine if I only have dxil.dll, and not dxcompiler.dll in the folder I have the Windows SDK installed
• Documentation about why users would/should use DXC, and how to get the .dlls
• Fix all the tests in github actions being broken because the .dlls are missing Currently defaults to using FXC.

Connections
Closes #2722
Builds off of, and closes #3147

Description
wgpu currently uses the old, slow, unmaintained FXC shader compiler for DX12. This allows use of Microsoft's newer faster shader compiler and also allows use of ShaderModel 6.0 instead of being limited to SM 5.1.

Testing
Tested both the FXC and DXC paths by running cargo nextest, as well as running the examples.

[cwfitzgerald]

Some comments:

Figure out why it's working on my machine if I only have dxil.dll, and not dxcompiler.dll in the folder

You probably have the windows sdk, which includes dxc.

Figure out how we should enable and disable DXC

I think we should have a feature at the wgpu-hal level, but not at the wgpu level. wgpu the crate unconditionally depends on it.

Figure out what the fallback should be if the user enables DXC, but the .dlls aren't available (crash with an error message? Fall back to dxc but log the error?)

We should warn in the case the user wants dxc, but the path isn't found, then fall back to FXC.

Figure out how we should enable and disable DXC, and whether that's at runtime/compiletime/startup/etc

I'm thinking of this enum sitting in an InstanceDescriptor:

enum Dx12Compiler {
    #[default]
    Fxc,
    Dxc { dxil_path: Option<String>, dxc_path: Option<String> },
}

Fix all the tests in github actions being broken because the .dlls are missing

We likely should default our tests to be fxc only, but with a flag they can enable DXC support. FF etc will be using fxc, so it's the most important path.

[codecov-commenter]

Codecov Report

Merging #3356 (2048c27) into master (0849e78) will increase coverage by 0.26%.
The diff coverage is 77.27%.

@@            Coverage Diff             @@
##           master    #3356      +/-   ##
==========================================
+ Coverage   65.23%   65.49%   +0.26%     
==========================================
  Files          86       66      -20     
  Lines       42772    37420    -5352     
==========================================
- Hits        27901    24508    -3393     
+ Misses      14871    12912    -1959     

Impacted Files	Coverage Δ
player/src/bin/play.rs	0.65% <0.00%> (-0.02%)	⬇️
wgpu-hal/src/lib.rs	24.03% <ø> (-2.89%)	⬇️
wgpu/src/context.rs	53.32% <ø> (ø)
wgpu/src/util/init.rs	68.25% <61.53%> (-1.75%)	⬇️
wgpu-core/src/hub.rs	64.90% <100.00%> (+3.77%)	⬆️
wgpu-core/src/instance.rs	68.17% <100.00%> (+2.09%)	⬆️
wgpu-info/src/main.rs	76.41% <100.00%> (ø)
wgpu-types/src/lib.rs	86.59% <100.00%> (-0.86%)	⬇️
wgpu/src/backend/direct.rs	56.90% <100.00%> (-0.09%)	⬇️
wgpu/src/lib.rs	51.30% <100.00%> (+0.26%)	⬆️
... and 32 more

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[Elabajaba]

A few changes, and a reminder for myself that I need to retest this on my laptop.

[cwfitzgerald]

This is really shaping up well. Got some comments, but nothing show stopping!

[Elabajaba]

I think everything should be cleaned up now.

[cwfitzgerald]

Amazing stuff!