i965/gs: Support UBOs by generating surface state and binding tables.

All but two of the piglit GLSL 1.50/uniform_buffer tests work, and
maxuniformblocksize and referenced-by-shader work.
!UPSTREAM: Expose GLSL 1.50 and GL 3.2 core contexts.

This lets me run tests normally, since globally doing GL version override
can break tests that require a compatibility context.
!UPSTREAM: Fix GS sampling

TODO:

- Seems like these functions ought to be rewritten to loop over
  program types.

- Handling of null gs seems clumsy.

- Can the same sampler be used in multiple shader stages (e.g. both VS
  and FS)?  If not, then it seems like we're going to run out of
  samplers in our bitfield.  If so, then is it guaranteed that
  fs->SamplerUnits[s] == gs->SamplerUnits[s]?  If so this needs an
  explanatory comment.  If not, this code seems broken.

- Decide what to do about code duplication.  Either I should de-dup
  the code or I suppose I should only update the gen7 code.

- Move to appropriate place in the series.