Replace gl_geom_attrib enum with gl_varying_slot.

Replace gl_geom_attrib enum with gl_varying_slot.

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

gl_geom_attrib -> gl_varying_slot
GEOM_ATTRIB_* -> VARYING_SLOT_*
GEOM_BIT_* -> VARYING_BIT_*

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