- rename ShaderInput variants
- rename feature flag
- hard error on Metal backend trying to compile SPIR-V
- fix test failing because of feature flag bits changing
While Naga checking is undoubtedly very useful, it currently lags behind
what is possible in SPIR-V and even what is promised by WGPU (ie binding
arrays). This adds an unsafe method to wgpu::Device to allow feeding
raw SPIR-V data into the backend, and adds a feature flag to request a
backend supporting this operation.