mesa: Add a flag to indicate whether a program uses gl_ClipDistance.

mesa: Add a flag to indicate whether a program uses gl_ClipDistance.

GLSL 1.30 requires us to use gl_ClipDistance for clipping if the
vertex shader contains a static write to it, and otherwise use
user-defined clipping planes.  Since the driver needs to behave
differently in these two cases, we need a flag to record whether the
shader has written to gl_ClipDistance.

The new flag is called UsesClipDistance.  We initially store it in
gl_shader_program (since that is the data structure that is available
when we check to see whethe gl_ClipDistance was written to), and we
later copy it to a flag with the same name in gl_vertex_program, since
that is a more convenient place for the driver to access it (in i965,
at least).

Reviewed-by: Eric Anholt <eric@anholt.net>
Tested-by: Brian Paul <brianp@vmware.com>