This allows two variables of the same struct type to be flattened
into the same interface struct without a member name conflict.
Add shaders-msl/frag/in_block_with_multiple_structs_of_same_type.frag
unit test shader to demonstrate this.
Implement this by flattening outputs and unflattening inputs explicitly.
This allows us to pass down a single struct instead of dealing with the
insanity that would be passing down each flattened member separately.
Remove stage_uniforms_var_id.
Seems to be dead code. Naked uniforms do not exist in SPIR-V for Vulkan,
which this seems to have been intended for. It was also unused elsewhere.
