`PipelineLayoutDescriptor`: make elements in `bind_group_layouts` optional

[reima]

Is your feature request related to a problem? Please describe.
Sometimes it is convenient to "skip" bind groups in a pipeline (see gpuweb/gpuweb#2043). PiplineLayoutDescriptor::bind_group_layouts does not allow skipping bind groups. The corresponding WebGPU type GPUPipelineLayoutDescriptor enables this by allowing null elements in bindGroupLayouts.

Describe the solution you'd like
Change the type of PipelineLayoutDescriptor::bind_group_layouts to &'a [Option<&'a BindGroupLayout>]. The semantics of None values should be the same as null in WebGPU.

Describe alternatives you've considered
What already works is supplying an "empty" bind group layout (without any entries) where a None is desired in the pipeline layout descriptor, and setting an "empty" bind group (without any entries) in the render pass.

Additional context
WebGPU has allowed null in GPUPipelineLayoutDescriptor.bindGroupLayouts since the Working Draft, 5 November 2024.

[reima]

I've only now discovered #1489, which is the same issue. Sorry for the noise.

[reima]

Upon closer inspection of #1489 I've noticed that while both issues are similar (gaps in bind groups indices), there is some difference:

• Gaps in binding group indices #1489 discusses using implicit pipeline layouts
• PipelineLayoutDescriptor: make elements in bind_group_layouts optional #7985 (this issue) discusses creating an explicit pipeline layout

I think this warrants reopening this issue, as they could possibly be addressed independently.