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.
glapidispatch.h exists so that core mesa (libmesa.a) can be built for
DRI drivers or for non-DRI drivers as a compile time decision (whether
IN_DRI_DRIVER is defined). It is of no use to glapi. This commit also
drops the use of glapidispatch.h in glx and libgl-xlib as they are
considered extensions to glapi when it comes to defining public GL
entries.