Backend drawing functions

[Fred Kiefer]

I rewrote the Bezel, Groove and Button drawing in NSGraphicsContext now
using NSDrawTiledRects. The old code is still there commented out, so
you can check more easily if it is still correct.
My test for the xgps back end look all right, the code is even some 5
percent faster. (But that is no real argument and may differ for
different benchmarks) It would be great, if somebody could also test and
benchmark the code on xdps to see if it is a good replacement for the
code there too. If so, I would like to drop all the optimized drawing
methods from NSGraphicsContext, as there would be no use for them any
more and go back to functions.
An open question here is, which of those functions should save and
restore there gstate. Currently they only do so in the xdps back end.
This is not very important, as long as we don't use PS operations mixed
with the normal drawing code. This is one more reason for going from PS
operations to the NSBezierPath methods, at least in applications.

Fred