- In MSL, declaring an array of matrix types like `threadgroup half4x4` causes
a 'no matching constructor' compilation error because MSL matrix types do not
have zero-argument default constructors and threadgroup variables cannot have
initializers.
- Fix this by declaring a POD `threadgroup half` array instead and casting
to `threadgroup half4x4 *` for matrix indexing.
Signed-off-by: JamePeng <jame_peng@sina.com>
