glsl: Implement a lowering pass for gl_ClipDistance.

In i965 GEN6+ (and I suspect most other hardware), gl_ClipDistance
needs to be laid out as a pair of vec4's (the first containing clip
distances 0-3, and the second containing clip distances 4-7).
However, it is declared in GLSL as an array of 8 floats.

This lowering pass acts at the GLSL level, modifying the declaration
of gl_ClipDistance so that it is an array of vec4's rather than an
array of floats, and renaming it to gl_ClipDistanceMESA.  In addition,
it modifies all accesses to the array so that they access the
appropiate component of one of the vec4's.

Since some hardware may not internally represent gl_ClipDistance as a
pair of vec4's, this lowering pass is optional.  To enable it, set the
LowerClipDistance flag in gl_shader_compiler_options to true.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/glsl/ir_optimization.h

@@ -69,6 +69,7 @@ bool lower_noise(exec_list *instructions);
 bool lower_variable_index_to_cond_assign(exec_list *instructions,
     bool lower_input, bool lower_output, bool lower_temp, bool lower_uniform);
 bool lower_quadop_vector(exec_list *instructions, bool dont_lower_swz);
+bool lower_clip_distance(exec_list *instructions);
 bool optimize_redundant_jumps(exec_list *instructions);
 
 ir_rvalue *