if their nominal Min and Max points differ.

This is a behavior change, but arguably a bug fix, as Eq wasn't
previously consistent with In, and the concept of a rectangle being a
set of points. This is demonstrated by the new geom_test.go test.

It does mean that r.Eq(s) no longer implies that Inset'ting both r and s
with a negative inset results in two rectangles that are still Eq, but
that seems acceptable to me.

The previous behavior is still available as "r == s".

Also clarify the image.Rect doc comment when the inputs are
non-canonical.

Also simplify the Point and Rectangle Eq implementations dating from
before Go 1.0, when you couldn't compare structs via the == operator.

Change-Id: Ic39e628db31dc5fe5220f4b444e6d5000eeace5b
Reviewed-on: https://go-review.googlesource.com/5006Reviewed-by: Rob Pike <r@golang.org>