image/draw: change argument type to be consistent with other args

Change-Id: Ic5b01c0a6912d87e812331d6525939df8f525cca
GitHub-Last-Rev: b6ffefcf8f2ec1ed7ee13f36e8fef6af965ba332
GitHub-Pull-Request: golang/go#29993
Reviewed-on: https://go-review.googlesource.com/c/go/+/160417Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/image/draw/draw.go

@@ -90,8 +90,8 @@ func clip(dst Image, r *image.Rectangle, src image.Image, sp *image.Point, mask
 	}
 }
 
-func processBackward(dst Image, r image.Rectangle, src image.Image, sp image.Point) bool {
-	return image.Image(dst) == src &&
+func processBackward(dst image.Image, r image.Rectangle, src image.Image, sp image.Point) bool {
+	return dst == src &&
 		r.Overlaps(r.Add(sp.Sub(r.Min))) &&
 		(sp.Y < r.Min.Y || (sp.Y == r.Min.Y && sp.X < r.Min.X))
 }