Fix rotated clip regions

bug: 26562461
fixes: 30109912

SeekBar clips out the area of the thumb when drawing its track. This
creates a clip region, since a rect has been subtracted.

This changes region transform support in computing final clip to
properly support any rect-to-rect transform (such as 90 degree rotation)
when applying a clip region.

Change-Id: Ib3bbc52637e59e00b3a7e7d5c4c40555e26c578b