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.