support for the DOUBLE-BUFFER extension to the Xprint server and DDX.
Additionally a "pixmap-scrubber" optimisation is added to the
PostScript DDX to remove all content from a vector pixmap when a
|PolyFillRect()| call covers the whole pixmap surface with a solid fill
(this avoids that a backbuffer vector pixmap gets constantly filled
even when the content is not visible anymore after |XdbeSwapBuffers()|
has cleared the hidden buffer with the background color).