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Fixes #8296.

Description

Addresses crashes when initializing a DX12 device that lacks support for mesh shaders.

This conditions creation of the command signatures for indirect mesh shader dispatch on the device supporting mesh shaders. The code here generally follows the patterns used in other backends (e.g.: Vulkan).

Testing

• Manually tested using cargo run --bin wgpu-examples hello_triangle (with WGPU_ADAPTER_NAME set to my Intel iGPU) that the hello_triangle example now runs properly.
• Manually tested using the mesh_shader example that mesh shaders still work with supported devices - set WGPU_ADAPTER_NAME to my dedicated GPU (which worked), and set it to my integrated GPU, which failed with Adapter does not support required features for this example: Features { features_wgpu: FeaturesWGPU(EXPERIMENTAL_MESH_SHADER), features_webgpu: FeaturesWebGPU(0x0) }.

Squash or Rebase?

Squash - will want to add a commit with changelog updates once we agree on impl.

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy --tests. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests.
• If this contains user-facing changes, add a CHANGELOG.md entry.