Fix crashes when the frag prog can't be handled in hardware (#11131)

Fix crashes when the frag prog can't be handled in hardware (#11131)

Must not change to/from swrast after Render.Start or bad things will happen.
(Driver will still somewhat incorrectly report an implementation error,
and apps can't really figure out if a prog is natively supported as validation
is later - could try doing it earlier to give some hint at least, even though
native status may still change later due to fog etc.)