The old method of using a different unpacked matrix type doesn't work
for scalar alignment. It certainly wouldn't have any effect for a square
matrix, since the number of columns and rows are the same. So now we'll
store them as arrays of packed vectors.
We used to use the Binding decoration for this, but this method is
hopelessly broken. If no explicit MSL resource remapping exists, we
remap automatically in a manner which should always "just work".
Structs are aligned as you would expect in MSL (maximum member
alignment), and it is not minimum 16 bytes like in std140.
Also rename the dummy "pad" members to a reserved naming scheme.
A lot of changes in spirv-opt output.
Some new invalid SPIR-V was found but most of them were not significant
for SPIRV-Cross, so just marked them as invalid.
Support MSL typedefs to declare 3-row row-major matrices as 3-column matrices.
Allow those matrices to be decorated as packed.
Support transposing those matrices when used.
Modify how member alignments are calculated.
