glsl: Move UsesClipDistance from gl_{vertex,geometry}_program into gl_program.

glsl: Move UsesClipDistance from gl_{vertex,geometry}_program into gl_program.

This will make it easier for back-ends to share code between geometry
shader and vertex shader compilation.  Also, it is renamed to
"UsesClipDistanceOut" to clarify that (a) in geometry shaders, it
refers to the gl_ClipDistance output rather than the gl_ClipDistance
input, and (b) it is irrelevant in fragment shaders.

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