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().

v3 (Bryan Cain <bryancain3@gmail.com>): fix indexing order of 2D
arrays.  Also, allow interpolation qualifiers in geometry shaders.
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.
glsl: Require geometry shader inputs to be arrays.
glsl/linker: Properly error check VS-GS linkage.

From the ARB_geometry_shader4 specification:

    A geometry shader may also read from an input varying variable declared
    with the "varying in" qualifiers. The value read will be the same value as
    written by the vertex shader for that varying variable. Since a geometry
    shader operates on primitives, each input varying variable needs to be
    declared as an array. Each element of such an array corresponds to a
    vertex of the primitive being processed. If the varying variable is
    declared as a scalar or matrix in the vertex shader, it will be a
    one-dimensional array in the geometry shader.

Therefore, a user-defined geometry shader input will always be an
array, and the *element type* of that array should match the type of
the corresponding vertex shader output.
glsl/linker: Properly pack GS input varyings.

Since geometry shader inputs are arrays (where the array index
indicates which vertex is being examined), varying packing needs to
treat them differently.
glsl: Modify varying packing to use a temporary exec_list.

This patch modifies lower_packed_varyings to store the packing code it
generates in a temporary exec_list, and then splice that list into the
shader's main() function when it's done.  This paves the way for
supporting geometry shader outputs, where we'll have to splice a clone
of the packing code before every call to EmitVertex().

As a side benefit, varying packing code is now emitted in the same
order for inputs and outputs; this should make debug output a little
easier to read.
glsl: Properly pack GS output varyings

In geometry shaders, outputs are consumed at the time of a call to
EmitVertex() (as opposed to all other shader types, where outputs are
consumed when the shader exits).  Therefore, when packing geometry
shader output varyings using lower_packed_varyings, we need to do the
packing at the time of the EmitVertex() call.

This patch accomplishes that by adding a new visitor class,
lower_packed_varyings_gs_splicer, which is responsible for splicing
the varying packing code into place wherever EmitVertex() is found.
st/mesa, glsl_to_tgsi: add support for geometry shaders

v2 (Bryan Cain <bryancain3@gmail.com>): fix 2D array indexing order.
mesa: fix transform feedback when a geometry shader is active
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
WIP: add GS visitors
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.