This adds the possibility to use the `gpu_Layer` builtin to
support layered rendering without geometry shader.
The `BuiltinBits::LAYER` builtin needs to be manually added
to the `GPUShaderCreateInfo` in order to use this feature.
Note: This is only available for shaders using `GPUShaderCreateInfo`.
A geometry shader fallback is generated if the extension
`AMD_shader_explicit_vertex_parameter` is not available.
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