CompilerMSL accesses options using same design pattern as CompilerGLSL and CompilerHLSL.
CompilerMSL support setting VA & rez binding specs via either constructor or compile() method overload.
CompilerMSL support single UBO packing and padding in single pass.
spriv_cross app (main.cpp) supports turning off UBO packing and padding via command line option.
Add MSL UBO alignment test shader.
