Implement shader barycentrics

[atlv24]

Description
Implements shader barycentrics for vk 1.3+ via VK_KHR_fragment_shader_barycentric BaryCoordKHR, dx12 via SV_Barycentrics, and metal 2.2 via barycentric_coord

Testing
There's a wgpu-gpu test.

Squash or Rebase?

Squash

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.

[inner-daemons]

Just going to link #2900 here, glad to see progress on this!

Also, #8342 is probably relevant, since I don't think this PR checks for DXC support before exposing barycentrics on DX12.

[cwfitzgerald]

Great stuff! Some small comments.

[cwfitzgerald]

Unfortunately lavapipe doesn't support barycentrics, weirdly.

Getting this when running it locally:

──── STDERR:             wgpu-test::wgpu-gpu [Executed] [Vulkan/NVIDIA GeForce RTX 4070 Laptop GPU/1] wgpu_gpu::shader_barycentric::barycentric
[2025-10-16T19:51:53Z ERROR wgpu_hal::vulkan::instance] VALIDATION [VUID-VkShaderModuleCreateInfo-pCode-08737 (0xa5625282)]
        vkCreateShaderModule(): pCreateInfo->pCode (spirv-val produced an error):
    Capability FragmentBarycentricKHR is not allowed by Vulkan 1.3 specification (or requires extension)
      OpCapability FragmentBarycentricKHR
    The Vulkan spec states: If pCode is a pointer to SPIR-V code, pCode must adhere to the validation rules described by the Validation Rules within a Module section of the SPIR-V Environment appendix (https://vulkan.lunarg.com/doc/view/1.4.321.0/windows/antora/spec/latest/chapters/shaders.html#VUID-VkShaderModuleCreateInfo-pCode-08737)
[2025-10-16T19:51:54Z ERROR wgpu_test::expectations] Validation Error: vkCreateShaderModule(): pCreateInfo->pCode (spirv-val produced an error):
    Capability FragmentBarycentricKHR is not allowed by Vulkan 1.3 specification (or requires extension)
      OpCapability FragmentBarycentricKHR
    The Vulkan spec states: If pCode is a pointer to SPIR-V code, pCode must adhere to the validation rules described by the Validation Rules within a Module section of the SPIR-V Environment appendix (https://vulkan.lunarg.com/doc/view/1.4.321.0/windows/antora/spec/latest/chapters/shaders.html#VUID-VkShaderModuleCreateInfo-pCode-08737)

thread '<unnamed>' panicked at tests\src\run.rs:121:9:
tests\tests\wgpu-gpu\shader_barycentric\mod.rs:33:44: test "wgpu_gpu::shader_barycentric::barycentric" did not behave as expected
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace