Fix Rectangle.Clip.
It could return a non-canonical rectangle if its arguments
did not overlap.
e.g. Rect(0, 0, 10, 10).Clip(Rect(0, 20, 10, 30)) -> Rect(0, 20, 10, 10)

R=rsc, r
CC=golang-dev, golang-dev
https://golang.org/cl/1290041