It could only be called from XMesaCopySubBuffer but this function is not used
by XFree86.
It seems that XMesaPutImageHelper would handle sub-images but never got
finished. Proper sub-image helpers should be written if need be.
Drop XMesaSetVisualDisplay(), XMesaReset(), no longer used.
Add XMesaCopyContext() and move the GlxSetRenderTables() call for XGL within
XMesaForceCurrent(). This is to make xserver/GL/mesa/X/xf86glx.c unaware of
Mesa internals.
Also, clean some ifdef's to make it clear that USE_XSHM and XFree86Server are
mutually exclusive.
Lastly,
- move gcstruct.h from glxheader.h up to xmesa_xf86.h since it calls *gc->ops
- drop GL/glxtokens.h from xm_api|dd.c, GLX tokens come from glcore.h and are
used irrelevant of XFree86.
