image: fix the overlap check in Rectangle.Intersect.

The doc comment for Rectangle.Intersect clearly states, "If the two
rectangles do not overlap then the zero rectangle will be returned."
Prior to this fix, calling Intersect on adjacent but non-overlapping
rectangles would return an empty but non-zero rectangle.

The fix essentially changes
if r.Min.X > r.Max.X || r.Min.Y > r.Max.Y { etc }
to
if r.Min.X >= r.Max.X || r.Min.Y >= r.Max.Y { etc }
(note that the > signs have become >= signs), but changing that line to:
if r.Empty() { etc }
seems clearer (and equivalent).

Change-Id: Ia654e4b9dc805978db3e94d7a9718b6366005360
Reviewed-on: https://go-review.googlesource.com/34853


Reviewed-by: David Crawshaw <crawshaw@golang.org>