glsl/gs: Fix transform feedback of gl_ClipDistance.

Since gl_ClipDistance is lowered from an array of floats to an array
of vec4's during compilation, transform feedback has special logic to
keep track of the pre-lowered array size so that attempting to perform
transform feedback on gl_ClipDistance produces a result with the
correct size.

Previously, this special logic always consulted the vertex shader's
size for gl_ClipDistance.  This patch fixes it so that it uses the
geometry shader's size for gl_ClipDistance when a geometry shader is
in use.

Fixes piglit test spec/glsl-1.50/transform-feedback-type-and-size.

v2: Change the type of LastClipDistanceArraySize to "unsigned", and
clarify the comment above it.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>

src/glsl/link_varyings.cpp

@@ -328,7 +328,7 @@ tfeedback_decl::assign_location(struct gl_context *ctx,
       const unsigned vector_elements =
          this->matched_candidate->type->fields.array->vector_elements;
       unsigned actual_array_size = this->is_clip_distance_mesa ?
-         prog->Vert.ClipDistanceArraySize :
+         prog->LastClipDistanceArraySize :
          this->matched_candidate->type->array_size();
 
       if (this->is_subscripted) {