Revert "image: fix the overlap check in Rectangle.Intersect."

This reverts commit a855da29.

Change-Id: I23c0351b0708877e0b3d1b44a2bc2799cee52cd1
Reviewed-on: https://go-review.googlesource.com/36426Reviewed-by: Russ Cox <rsc@golang.org>

src/image/geom.go

@@ -161,11 +161,7 @@ func (r Rectangle) Intersect(s Rectangle) Rectangle {
 	if r.Max.Y > s.Max.Y {
 		r.Max.Y = s.Max.Y
 	}
-	// Letting r0 and s0 be the values of r and s at the time that the method
-	// is called, this next line is equivalent to:
-	//
-	// if max(r0.Min.X, s0.Min.X) >= min(r0.Max.X, s0.Max.X) || likewiseForY { etc }
-	if r.Empty() {
+	if r.Min.X > r.Max.X || r.Min.Y > r.Max.Y {
 		return ZR
 	}
 	return r

src/image/geom_test.go

@@ -28,7 +28,6 @@ func TestRectangle(t *testing.T) {
 
 	rects := []Rectangle{
 		Rect(0, 0, 10, 10),
-		Rect(10, 0, 20, 10),
 		Rect(1, 2, 3, 4),
 		Rect(4, 6, 10, 10),
 		Rect(2, 3, 12, 5),
@@ -63,9 +62,9 @@ func TestRectangle(t *testing.T) {
 			if err := in(a, s); err != nil {
 				t.Errorf("Intersect: r=%s, s=%s, a=%s, a not in s: %v", r, s, a, err)
 			}
-			if isZero, overlaps := a == (Rectangle{}), r.Overlaps(s); isZero == overlaps {
-				t.Errorf("Intersect: r=%s, s=%s, a=%s: isZero=%t same as overlaps=%t",
-					r, s, a, isZero, overlaps)
+			if a.Empty() == r.Overlaps(s) {
+				t.Errorf("Intersect: r=%s, s=%s, a=%s: empty=%t same as overlaps=%t",
+					r, s, a, a.Empty(), r.Overlaps(s))
 			}
 			largerThanA := [4]Rectangle{a, a, a, a}
 			largerThanA[0].Min.X--