When flattening shader inputs for stage_in, which are to be read from a buffer
that was populated as nested structs during an earlier stage, the structs will
be aligned according to C++ rules, which can affect the alignment of the first
member of the flattened input struct.
Add SPIRVShaderOutput::firstStructMemberAlignment to track the alignment
requirements of the first member of a nested structure, and recursively
determine the alignment of the first member of each nested output structure.
Move sizeOfOutput() from MVKPipeline.mm to SPIRVReflection.h,
rename to getShaderOutputSize(), and add getShaderOutputAlignment()
to extract member alignment.
afd997ab31