Add a facility to route all rasterization through a fragment program

Add a facility to route all rasterization through a fragment program
which is automatically generated to match the current texture environment
state.  Introduces a new value ctx->FragmentProgram._Active which is
true when either _Enabled is true or there is such a fragment program
ready to run.

To test out on a driver running the software rasterizer, set
MESA_TEX_PROG=t in the environment.  It goes without saying that performance
is lower for the software rasterizer in this mode.