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.