glsl: During linking, record whether a GS uses EndPrimitive().

This information will be useful in the i965 back end.
i965/gs: Set control data header size/format appropriately for EndPrimitive().

The gen7 geometry shader uses a "control data header" portion of the
output URB entry to store either

(a) flag bits (1 bit/vertex) indicating whether EndPrimitive() was
    called after each vertex, or

(b) stream ID bits (2 bits/vertex) indicating which stream each vertex
    should be sent to (when multiple transform feedback streams are in
    use).

This patch modifies do_vec4_gs_prog() to determine the correct
hardware settings for configuring the control data header, and
modifies upload_gs_state() to propagate these settings to the
hardware.

In addition, it modifies do_vec4_gs_prog() to ensure that the output
URB entry is large enough to contain both the output vertices *and*
the control data header.
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.
i965/gen7: Add the ability to send URB_WRITE_OWORD messages.

This will be used by geometry shaders to implement EndPrimitive()
functionality.
i965/gs: Add opcodes needed for EndPrimitive().
i965/vec4: Add the ability to emit opcodes with just a dst register.

This is needed for GS_OPCODE_PREPARE_CHANNEL_MASKS.
i965/gs: implement EndPrimitive() functionality in the visitor.
!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.
!UPSTREAM: GS sampling fixes 1
!UPSTREAM: GS sampling fixes 2

TODO:
- Move later
!UPSTREAM: Fix GS sampling.

TODO:

- Need to re-examine after Ken's re-work

- 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.
!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.
main: Expose geometry shader constants in GL 3.0.
!UPSTREAM: WIP: add gl_in to builtin_variables.cpp.