mesa: Allow contexts of different APIs to coexist.

mesa: Allow contexts of different APIs to coexist.

This effectively redoes 1741ddb747ca0be284315adb4b6fe67ddf292d03 in a
way that allows contexts of different APIs to coexist.

First, the changes to the remap table are reverted.  The remap table
(driDispatchRemapTable) is always initialized in the same way regardless
of the context API.

es_generator.py is updated to use a local remap table, whose sole
purpose is to help initialize its dispatch table.  The local remap table
and the global one are always different, as they use different
glapidispatch.h.  But the dispatch tables initialized by both remap
tables are always compatible with glapi (libGL.so).

Finally, the semantics of one_time_init are changed to per-api one-time
initialization.