SDL backend

[Christopher Armstrong]

First of all, a thanks to all the hard work of the gnustep people over
the years. It seems this project is finally looking more usable and
stable than it has done in the past, and I'm finally getting the hang of
the API.

Secondly, I have been wondering whether or not an SDL backend for
gnustep has been considered. Is this considered feasible. I have been
looking into this myself, but the interaction between GSGState and
GSContext classes is something I've still to get my head around. I've
been looking at it as a more proof of concept thing, but SDL is quite
portable, and it may help the AppKit move to other platforms if
desired. 

There are some support libraries for image transformations and rotations
as well as graphics primitives (circle, arc, rectangle, line etc.) as
well as some builtin alpha blending/blitting support.

Christopher Armstrong