Replace gl_geom_result enum with gl_varying_slot.

Replace gl_geom_result enum with gl_varying_slot.

This patch makes the following search-and-replace changes:

gl_geom_result -> gl_varying_slot
GEOM_RESULT_* -> VARYING_SLOT_*

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Tested-by: Brian Paul <brianp@vmware.com>