Add GS_OPCODE_SET_WRITE_OFFSET
WIP: add GS visitors
Make some files safe to include from C
WIP: make the state atom for Gen7 geometry shaders.  TODO: maybe disentangle
WIP: generalize DEBUG_VS for GS
WIP: turn on GS, and mark GS stuff as untested for safety
!UPSTREAM: Generalize some error messages for GS.

TODO: find more places to generalize.
TODO: Don't use "VS/GS" terminology.  Use "vec4" terminology, since that will endure longer.