glapioffsets.h exists for the same reason as glapidispatch.h does. It
is of no use to glapi. This commit also drops the use of glapioffsets.h
in glx as glx is considered an extension to glapi when it comes to
defining public GL entries.
Specifically, move all or most of
glapi/glapi.c to mapi/u_current.c,
glapi/glapi_execmem.c to mapi/u_execmem.c,
glapi/glthread.[ch] to mapi/u_thread.[ch]
and remove their dependencies on core Mesa headers.
