Remove segfaults in WebGPU threads
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I have also increased the number of staging buffers for presentation.
Segfault occurred at 2 places-
1. RenderPipeline descriptor.
2. BufferMapAsync callback.
r?@kvark
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Implement GPUTexture and GPUTextureView
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This also include changes to CodegenRust.py to allow enum values starting with digits.
r?@kvark
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This is based on compiling with `RUSTFLAGS="-W unused_crate_dependencies"` (CC https://github.com/rust-lang/rust/pull/72342) in a recent Nightly (more so than used in the tree as of this writing, CC https://github.com/servo/servo/issues/26661 for work-arounds).
Only one crate is actually removed from the dependency graph, others are still dependended from other places.
Changed the Requests/Response from tuples to named struct variants and also sorted in alphabetical order.
Replaced the ID generator functions from `globalscope` with a single function,
which returns a `RefMut` and can call the appropriate method to generate resource IDs.
Implemented the `mapReadAsync` and fixed the `unmap` functions of `GPUBuffer`.
Added `mapped` internal slot for tracking the ArrayBuffer/Promise.
Added more states to the `GPUBufferState` enum.
