quartz: Emulate root window as 1x1 bitmap

quartz: Emulate root window as 1x1 bitmap

We subclass GdkWindowImplQuartz into a new GdkRootWindowImplQuartz,
and override the get_context method in order to do this cleanly.
Also made release_context a virtual method, since the root window has
to release its CGContextRef differently compared to normal windows.