to support InvertRect() for the XOR-ing of rectangles. Makes selection
work as expected on windows and unix as opposed to the rectangle hack.
Stubs added for all other platforms.
added LockDrawingSurface() and UnlockDrawingSurface() to rendering
context.
fixed state maintenance errors in rendering context win.
added IsReleaseDCDestructive() to drawingsurfacewin.
implementation. cleaned up windows blending code to the point where we
don't really need platform specific blending implementations at all
anymore. added region area return from nsIRegion::GetRects(). unified all
windows bitmap management code under nsDrawingSurfaceWin.cpp.
