source files: xgtm.[ch], term/*.trm, gtplot.h
(GT-GET-ZBUFFERING-IS-ON) (SEND <gtm:terminal> :GET-ZBUFFERING-IS-ON)
Returns NIL iff zbuffering is off. If zbuffering is on, foreground things will automatically obscure background things, no matter which order they are drawn in. If zbuffering is off, the last thing drawn overwrites the other things, no matter which is closer.