See https://github.com/gfx-rs/wgpu/issues/4915
Also:
* Remove unused vert-out frag-in variables from shaders (naga doesn't
like this probably because they are optimized out on the fragment
side). This restriction from naga may be relaxed in the future
see https://github.com/gfx-rs/wgpu/issues/3748.
* Enable OptimizationLevel::Performance for shaderc by default
* Add a environment variable VOXYGEN_SHADERC_OPTS for disabling this
(e.g. to test if it actually makes a difference on any platform).
(TODO: testing might be easier if there was a way to do toggle it
without restarting...)
