image/draw: fix double-draw when the dst is paletted.

The second (fallback) draw is a no-op, but it's a non-trivial amount of work.

Fixes #11550.

benchmark               old ns/op     new ns/op     delta
BenchmarkPaletted-4     16301219      7309568       -55.16%

Change-Id: Ic88c537b2b0c710cf517888f3dd15cb702dd142f
Reviewed-on: https://go-review.googlesource.com/11858Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/image/draw/bench_test.go

@@ -7,6 +7,7 @@ package draw
 import (
 	"image"
 	"image/color"
+	"reflect"
 	"testing"
 )
 
@@ -15,6 +16,11 @@ const (
 	srcw, srch = 400, 300
 )
 
+var palette = color.Palette{
+	color.Black,
+	color.White,
+}
+
 // bench benchmarks drawing src and mask images onto a dst image with the
 // given op and the color models to create those images from.
 // The created images' pixels are initialized to non-zero values.
@@ -50,8 +56,20 @@ func bench(b *testing.B, dcm, scm, mcm color.Model, op Op) {
 		}
 		dst = dst1
 	default:
+		// The == operator isn't defined on a color.Palette (a slice), so we
+		// use reflection.
+		if reflect.DeepEqual(dcm, palette) {
+			dst1 := image.NewPaletted(image.Rect(0, 0, dstw, dsth), palette)
+			for y := 0; y < dsth; y++ {
+				for x := 0; x < dstw; x++ {
+					dst1.SetColorIndex(x, y, uint8(x^y)&1)
+				}
+			}
+			dst = dst1
+		} else {
 			b.Fatal("unknown destination color model", dcm)
 		}
+	}
 
 	var src image.Image
 	switch scm {
@@ -218,6 +236,10 @@ func BenchmarkRGBA(b *testing.B) {
 	bench(b, color.RGBAModel, color.RGBA64Model, nil, Src)
 }
 
+func BenchmarkPaletted(b *testing.B) {
+	bench(b, palette, color.RGBAModel, nil, Src)
+}
+
 // The BenchmarkGenericFoo functions exercise the generic, slow-path code.
 
 func BenchmarkGenericOver(b *testing.B) {

src/image/draw/draw.go

@@ -176,6 +176,7 @@ func DrawMask(dst Image, r image.Rectangle, src image.Image, sp image.Point, mas
 	case *image.Paletted:
 		if op == Src && mask == nil && !processBackward(dst, r, src, sp) {
 			drawPaletted(dst0, r, src, sp, false)
+			return
 		}
 	}