mesa: new _mesa_set_vp_override() function for driver-override of vertex program

mesa: new _mesa_set_vp_override() function for driver-override of vertex program

Patch provide by Keith.
Used in state tracker by DrawPixels to indicate that the state tracker (driver)
is using its own vertex program.  This prevents the texenvprogram code from
replacing conventional shader inputs with state vars.
Fixes glDraw/CopyPixels regressions.