Reimplement CGContextDrawImage in terms of _CGContextDrawImageRect.
Reimplement CGContextDrawImage in terms of _CGContextDrawImageRect.

This fixes the heavy drawing overhead on 9patch images, which used to
apply a context-global clip and draw the image in full nine separate
times.

Now we get to use D2D's native image region subsetting support!

bbowman/WinObjC,ehren/WinObjC,bbowman/WinObjC,bbowman/WinObjC,nathpete-msft/WinObjC,bbowman/WinObjC,nathpete-msft/WinObjC,nathpete-msft/WinObjC,nathpete-msft/WinObjC,pradipd/WinObjC,pradipd/WinObjC,ehren/WinObjC,ehren/WinObjC,bbowman/WinObjC,pradipd/WinObjC,nathpete-msft/WinObjC,ehren/WinObjC,ehren/WinObjC,pradipd/WinObjC,pradipd/WinObjC