* Add new clipping API for rectangles (ClipToRect, ClipToInverseRect)
and shapes (ClipToShape, ClipToInverseShape)
* Works with affine transforms and automatically switches from fast
region-based clipping to alpha-mask based clipping as necessary.
* Always self-intersecting, i.e. no state push required to further
narrow down the clipping region. All of the 4 new methods can be
mixed in any order.
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