This is similar to the Event.Bounds method. Some functions in other
packages take an image.Rectangle (i.e. bounds), others take an
image.Point (i.e size).
Change-Id: Ie892a8902992d4e6b9edfbf6d12342297fae4068
Reviewed-on: https://go-review.googlesource.com/14003
Reviewed-by: David Crawshaw <crawshaw@golang.org>