run-wasm helper (cargo-run-wasm) for testing the wgpu runner on the web.

[eddyb]

Opening as draft because this is blocked on wgpu::backend::web being completely broken:

• Dynamic dispatch layer breaks on web gfx-rs/wgpu#3430
  • _{the short version is that they made a Copy type convertible to a type that implement Drop, leading to a kind of UAF ("use after free") but specifically in the context of wasm-bindgen's Rust->JS value ownership}

TODO(@eddyb): consider hot reloading.

EDIT: blocked on:

• example-runner-wgpu: add --force-spirv-passthru and rely on it for debugPrintf. #1036

---

EDIT: this is now possible thanks to:

• Avoid using WasmAbi functions on WebGPU backend gfx-rs/wgpu#3657

Had to work around some sRGB complications (and push constants) but it finally works!

• cargo run-wasm --package example-runner-wgpu --bin wgpu_runner
  • (defaults to serving the result of the build on http://localhost:8000)
• google-chrome-unstable --enable-features=Vulkan --enable-unsafe-webgpu http://localhost:8000

[eddyb]

I added PushConstant-as-read-only-StorageBuffer emulation, and using that and #1018 I get: