Expose pipeline constants to materials

[Visse]

Objective

Allow materials to use pipeline constants (pipeline-overridable constants).
They are already available in wgpu, but bevy didn't expose them.

Solution

Expose constants in RenderPipelineDescriptor & ComputePipelineDescriptor, allowing materials to specify them in their specilize function.

Note: I had to remove the Eq derive from ComputePipelineDescriptor, VertexState and FragmentState, due to the new f64 field. It was already a bit inconsistent with RenderPipelineDescriptor not having it.

Testing

• Ran cargo check
• Created an example & ran it
• Couldn't run cargo test due to it taking looots of disk space to run, but I have a hard time seeing it break something at runtime

Showcase

See the added example, where pipeline constants are used to change the LEVELS override in WGSL.

[github-actions]

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[coreh]

Is this restricted to just f64 -> f32? Is the f64 there because the WebGPU is made with JS in mind? Is there some sort of internal conversion for other types?

Could we expose some sort of PipelineConstant newtype so you could pass other types, or would that be too silly since they will be coerced to f64 behind the scenes anyway?

[beicause]

Is this restricted to just f64 -> f32? Is the f64 there because the WebGPU is made with JS in mind? Is there some sort of internal conversion for other types?

Could we expose some sort of PipelineConstant newtype so you could pass other types, or would that be too silly since they will be coerced to f64 behind the scenes anyway?

It is f64 like LoadOp::Clear values.The conversion rule is https://gpuweb.github.io/gpuweb/#conversions-to-wgsl

[beicause]

Some unnecessary cloning, although it's not big deal since these constants are empty.