i965/gs: Create brw_context::vec4_gs structure to track GS program state.
!UPSTREAM: 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.

TODO:
- maybe squash with prev stuff
i965/gs: make the state atom for compiling Gen7 geometry shaders.
!UPSTREAM: i965/gs: Generalize some error messages for GS.

TODO: find more places to generalize.
TODO: Don't use "VS/GS" terminology.  Use "vec4" terminology, since that will endure longer.
i965/gs(gen7): Allocate URB space for use by geometry shader.
!UPSTREAM: i965/gs(gen7): Allocate push constant space.
i965/vs: generalize push const upload in preparation for GS.
i965/gs: set up GS state.
!UPSTREAM ===== END of preliminary back-end work =====
!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.
!UPSTREAM: Primitive ID part 1.
!UPSTREAM: Primitive ID part 2.
!UPSTREAM: Primitive ID part 3.
!UPSTREAM: Primitive ID part 4.
i965/gs: Fixup gl_PointSize on entry to geometry shaders.

gl_PointSize is stored in the w component of VARYING_SLOT_PSIZ, but
the geometry shader infrastructure assumes that it should look for all
geometry shader inputs of type float in the x component.  So when
compiling a geomtery shader that uses a gl_PointSize input, fix it up
during the shader prolog by moving the x component to the w component.

This is similar to how we emit fixups and workarounds for vertex
shader attributes.
i965/gen7: Allow URB_WRITE channel masks to be used.

Previously, brw_urb_WRITE() would unconditionally override the channel
masks in the URB_WRITE message to 0xff (indicating that all channels
should be written to the URB).

For geometry shaders, we need the ability to set the channel masks
programatically, so create a flag to prevent brw_urb_WRITE() from
overriding them.
!UPSTREAM: i965/gen7: Add the ability to send URB_WRITE_OWORD messages.

This will be used by geometry shaders to implement EndPrimitive()
functionality.

TODO:
- Move to appropriate place in series.
!UPSTREAM: i965/gs: implement EndPrimitive() functionality.

Stuff that needs fixing:
- Probably should pack vertex_count and cut_bits into a single register
- Code should be conditioned on whether the shader uses EndPrimitive()
!UPSTREAM: i965: Add new primitive types.

TODO: test.
!UPSTREAM: glsl: Add new geometry shader constants.

TOOD:
- These constants need to be plumbed through to the driver back-ends.