Implement enough of ARB_fragment_program for Keith's texenv program

Implement enough of ARB_fragment_program for Keith's texenv program
generation in Mesa.  Requires most recent Mesa cvs to work correctly.

Swizzle code is possibly slow/buggy and will probably be replaced.

Old behaviour can be controlled by a #define in r300_context.h, if
no-one minds I'll remove the old code later on.

Also, disabled cube map extension.  AFAIK We don't support it yet,
and it causes issues with UT2004.