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.
!UPSTREAM ===== BEGIN rebase of calim's GS patches =====
draw/gs: fix allocation of buffer for GS output vertices

Before, it accounted for the size of the vertices but not the other fields
in the vertex_header struct, which caused memory corruption.
mesa: add EXT_geometry_shader4 and ARB_geometry_shader4 to extension list
mesa: account for geometry shader texture fetches in update_texture_state
glcpp: define extension macros for EXT/ARB_geometry_shader4
glsl: add builtins for EXT/ARB_geometry_shader4
glsl: add ir_emitvertex and ir_endprim instruction types

These correspond to the EmitVertex and EndPrimitive functions in GLSL.

v2 (Paul Berry <stereotype441@gmail.com>): Add stub implementations of
new pure visitor functions to i965's vec4_visitor and fs_visitor
classes.
glsl: lower gl_ClipDistanceIn to gl_ClipDistanceInMESA in lower_clip_distance

This corresponds to the lowering of gl_ClipDistance to gl_ClipDistanceMESA.
It converts the 2D array gl_ClipDistanceIn from EXT/ARB_geometry_shader4 to
a 1D array of vec4s, gl_ClipDistanceInMESA.
glsl: support compilation of geometry shaders via EXT/ARB_geometry_shader4

This commit adds all of the parsing and semantics for EXT/ARB_geometry_shader4,
including support for 2D arrays as inputs.

v2 (Paul Berry <stereotype441@gmail.com>): Adjust i965's
brw_link_shader() to pass the new is_geometry_shader argument to
do_set_program_inouts.  Add a few missing calls to
get_pipeline_stage().  Fix some signed/unsigned comparison warnings.
Fix handling of NULL consumer in assign_varying_locations().
mesa: Refactor copying of linked program data.

This patch creates a single function to copy the the UsesClipDistance
flag from gl_shader_program.Vert to gl_vertex_program.  Previously
this logic was duplicated in the i965-specific function
brw_link_shader() and the core mesa function _mesa_ir_link_shader().

This logic will have to be expanded to support geometry shaders, and I
don't want to have to update it in two separate places.
mesa: Copy linked program data for GS.

The documentation for gl_shader_program.Geom and gl_geometry_program
says that the former is copied to the latter at link time, but this
wasn't happening.  This patch causes _mesa_ir_link_shader() to perform
the copy, and updates comment accordingly.
st/mesa, glsl_to_tgsi: add support for geometry shaders

Note (Paul Berry <stereotype441@gmail.com>): I'm not sure I rebased
this correctly.  It appears that some amount of 2D array support was
added to tgsi after the patch was originally written.
WIP: make stuff safe to include from C++
i965: Stop including brw_vs.h from brw_vec4.h

This is backwards from what we are going to want in the long term, which is:

- brw_vec4.h declares general-purpose vec4 infrastructure needed by
  both VS and GS
- brw_vs.h includes brw_vec4.h and adds VS-specific parts.
- brw_gs.h includes brw_vec4.h and adds GS-specific parts.

Note that at the moment brw_vec.h contains a fair amount of
VS-specific declarations--I plan to address that in a later patch.
WIP: i965: Add an opcode for doing a URB write with an offset from a register (needed for GS).
Add brw->geometry_program
Add GS_OPCODE_THREAD_END
Add GS_OPCODE_SET_WRITE_OFFSET
Add GS_OPCODE_SET_VERTEX_COUNT