nvc0: implement point coord replacement

nvc0: implement point coord replacement

But we have to cheat and peek at the GENERIC semantic indices the
state tracker uses for TEXn.
Only outputs from 0x300 to 0x37c can be replaced, and so we have to
know on shader compilation which ones to put there in order to keep
doing separate shader objects properly.

At some point I'll probably create a patch that makes gallium not
force us to discard the information about what is a TexCoord.