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 <[email protected]>

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