Opcode decoding: 0xC0 isn't a valid command.

Fix our opcode decoders to handle this appropriately.
OGL/VertexManager: Remove unnused m_CurrentVertexFmt
remove unused and unexported function
Merge pull request #354 from magumagu/gx-primitive-mask

Opcode decoding: 0xC0 isn't a valid command.
Merge pull request #366 from Jezze/vertex

OGL/VertexManager: Remove unused variable.
Merge pull request #375 from degasus/remove_SkipVertices

VertexLoaderManager: Remove unused function.
Video backends: fix behavior of "konst" TEV inputs.

Also fixes TEVCOLORARG_HALF.

Values verified to match hardwarere.