Reflect DX10-style output signatures

This handles the OSGN fourCC which represents the information in
D3D10_SIGNATURE_PARAMETER_DESC.

DX11 (OSG5 fourCC) has some extra data. SM5 shaders seem to still use
OSGN if they doe not need the extra data.

Also fixed decoding of Mask and added ReadWriteMask for input and output
signatures.