i965/vs: Generalize vertex emission code in preparation for GS.

This patch introduces a new function, vec4_visitor::emit_vertex(),
which contains the code for emitting vertices that will need to be
common between the vertex and geometry shaders.
i965/vs: Generalize attribute setup code in preparation for GS.

This patch introduces a new function,
vec4_visitor::lower_attributes_to_hw_regs(), which replaces registers
of type ATTR in the instruction stream with the hardware registers
that store those attributes.  This logic will need to be common
between the vertex and geometry shaders.
i965/vs: Generalize computation of array strides in preparation for GS.

Geometry shader inputs are arrays, but they use an unusual array
layout: instead of all array elements for a given geometry shader
input being stored consecutively, all geometry shader inputs are
interleaved into one giant array.  As a result, the array stride we
use to access geometry shader inputs must be equal to the size of the
input VUE, rather than the size of the array element.

This patch introduces a new virtual function,
vec4_visitor::compute_array_stride(), which will allow geometry shader
compilation to specialize the computation of array stride to account
for the unusual layout of geometry shader input arrays.  It also
renames the local variable that the ir_dereference_array visitor uses
to store the stride, to avoid confusion.
i965: Generalize computation of VUE map in preparation for GS.

This patch modifies the arguments to brw_compute_vue_map() so that
they no longer bake in the assumption that we are generating a VUE map
for vertex shader outputs.  It also makes the function non-static so
that we can re-use it for geometry shader outputs.
Add brw->geometry_program
Add GS_OPCODE_THREAD_END
Add GS_OPCODE_SET_WRITE_OFFSET
Add GS_OPCODE_SET_VERTEX_COUNT
WIP: add GS visitors
BLARG
Make some files safe to include from C
i965: Make prim_to_hw_prim accessible outside brw_draw.c.

We will need access to this array in order to configure the geometry
shader.
i965/gs(gen7.5): Set max_gs_threads correctly
WIP: make the state atom for compiling Gen7 geometry shaders.  TODO: maybe disentangle
WIP: generalize DEBUG_VS for GS
!UPSTREAM: 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