Clean and update XMesa/XFree86 interface.

Clean and update XMesa/XFree86 interface.

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.