Store clipping distance for user clip planes as part of vertex processing

Store clipping distance for user clip planes as part of vertex processing

Once the clipping distance is calculated and stored per vertex, the
distances can be re-used when clipping is actually performed.  This
doesn't have any immediate benefit, but it paves the way for
implementing gl_ClipDistance in vertex shaders and result.clip[] in
vertex programs.

This has not produces any oglconform regressions on my G31 system
which uses software TNL.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>