* back/Source/gsc/GSContext.m
        * back/Source/cairo/CairoContext.m
        * back/Source/art/ARTContext.m
        * back/Source/xlib/XGContext.m
        * back/Source/winlib/WIN32Context.m: Move the bitmap conversion
        code from cairo to the gsc class. Add a new method to detect
        incompatible image formats and implement that for all backends.
        * back/Source/winlib/WIN32GState.m (GSCreateBitmap): Improve the
        handling of device black/white colour space.


git-svn-id: svn+ssh://svn.gna.org/svn/gnustep/libs/back/trunk@36025 72102866-910b-0410-8b05-ffd578937521