i965 Gen6: Implement gl_ClipVertex.

i965 Gen6: Implement gl_ClipVertex.

This patch implements proper support for gl_ClipVertex by causing the
new VS backend to populate the clip distance VUE slots using
VERT_RESULT_CLIP_VERTEX when appropriate, and by using the
untransformed clip planes in ctx->Transform.EyeUserPlane rather than
the transformed clip planes in ctx->Transform._ClipUserPlane when a
GLSL-based vertex shader is in use.

When not using a GLSL-based vertex shader, we use
ctx->Transform._ClipUserPlane (which is what we used prior to this
patch).  This ensures that clipping is still performed correctly for
fixed function and ARB vertex programs.  A new function,
brw_select_clip_planes() is used to determine whether to use
_ClipUserPlane or EyeUserPlane, so that the logic for making this
decision is shared between the new and old vertex shaders.

Fixes the following Piglit tests on i965 Gen6:
- vs-clip-vertex-const-accept
- vs-clip-vertex-const-reject
- vs-clip-vertex-different-from-position
- vs-clip-vertex-equal-to-position
- vs-clip-vertex-homogeneity
- vs-clip-based-on-position
- vs-clip-based-on-position-homogeneity
- clip-plane-transformation clipvert_pos
- clip-plane-transformation pos_clipvert
- clip-plane-transformation pos

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chad Versace <chad@chad-versace.us>