[cogl-clip-stack] Support pushing rectangles onto the stack using window coords

[cogl-clip-stack] Support pushing rectangles onto the stack using window coords

Previously clipping could only be specified in object coordinates, now
rectangles can also be pushed in window coordinates.

Internally rectangles pushed this way are intersected and then clipped using
scissoring.  We also transparently try to convert rectangles pushed in
object coordinates into window coordinates as we anticipate the scissoring
path will be faster then the clip planes and undoubtably it will be faster
than using the stencil buffer.