image/draw: add an Op argument to Draw.

R=r
CC=golang-dev
https://golang.org/cl/4622041

src/pkg/image/draw/draw.go

@@ -34,9 +34,9 @@ type Image interface {
 	Set(x, y int, c image.Color)
 }
 
-// Draw calls DrawMask with a nil mask and an Over op.
-func Draw(dst Image, r image.Rectangle, src image.Image, sp image.Point) {
-	DrawMask(dst, r, src, sp, nil, image.ZP, Over)
+// Draw calls DrawMask with a nil mask.
+func Draw(dst Image, r image.Rectangle, src image.Image, sp image.Point, op Op) {
+	DrawMask(dst, r, src, sp, nil, image.ZP, op)
 }
 
 // clip clips r against each image's bounds (after translating into the

src/pkg/image/draw/draw_test.go

@@ -271,7 +271,7 @@ func TestNonZeroSrcPt(t *testing.T) {
 	b.Set(1, 0, image.RGBAColor{0, 0, 5, 5})
 	b.Set(0, 1, image.RGBAColor{0, 5, 0, 5})
 	b.Set(1, 1, image.RGBAColor{5, 0, 0, 5})
-	Draw(a, image.Rect(0, 0, 1, 1), b, image.Pt(1, 1))
+	Draw(a, image.Rect(0, 0, 1, 1), b, image.Pt(1, 1), Over)
 	if !eq(image.RGBAColor{5, 0, 0, 5}, a.At(0, 0)) {
 		t.Errorf("non-zero src pt: want %v got %v", image.RGBAColor{5, 0, 0, 5}, a.At(0, 0))
 	}