i965: Create a pointer in brw_context to the geometry output VUE map.

Currently, the GPU pipeline has one active VUE map in effect at any
given time--the one representing the layout of vertex data coming from
the vertex shader.  However, when geometry shaders are added, they
will have their own independent VUE map.  Later pipeline stages (clip,
sf, fs) will need to consult the geometry shader VUE map if a geometry
shader is in use, and the vertex shader VUE map otherwise.

This patch adds a new field to brw_context, vue_map_geom_out, which
points to whichever VUE map should be used by later pipeline stages.
It also adds a new state flag, BRW_NEW_VUE_MAP_GEOM_OUT, which is
signalled whenever this pointer changes.

Since we don't support geometry shaders yet, vue_map_geom_out is
currently set only by the brw_vs_prog state atom.
i965: Use brw.vue_map_geom_out instead of VS output VUE map where appropriate.

This patch modifies post-GS pipeline stages (transform feedback, clip,
sf, fs) to refer to the VUE map through brw->vue_map_geom_out rather
than brw->vs.prog_data->vue_map.  This ensures that when geometry
shader support is added, these pipeline stages will consult the
geometry shader output VUE map when appropriate, rather than the
vertex shader output VUE map.
i965/fs: Rename vp_outputs_written to input_slots_valid.

With the introduction of geometry shaders, fragment inputs will no
longer come exclusively from the vertex shader; sometimes they come
from the geometry shader.  So the name "vp_outputs_written" will
become a misnomer.  This patch renames vp_outputs_written to
input_slots_valid, to reflect the true meaning of the bitfield from
the fragment shader's point of view: it indicates which of the
possible input slots contain valid data that was written by the
previous shader stage.
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.
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 vec4_visitor::vp more generic
WIP: clean up uses of prog_data.
WIP: split brw_vs_compile out to brw_vec4_compile
WIP: split brw_vs_prog_key
WIP: split brw_vs_prog_data
WIP: make vec4_vs_visitor class
WIP: extract virtual functions from ir_variable visitor

Includes making a new function to handle variables of type
ir_var_system_value, which differ in VS and GS.

The following functions have been renamed to reflect their generic
purpose, rather than their VS-specific behaviour:

- emit_attribute_fixups() -> emit_prolog()
- emit_vertex_program_code() -> emit_program_code()
- emit_urb_writes() -> emit_thread_end()
WIP: move VS-specific functions to vec4_vs_visitor