restorable: Avoid memory allocating when an entire image is cleared

internal/restorable/image.go

@@ -169,10 +169,16 @@ func (i *Image) ReplacePixels(pixels []byte, x, y, width, height int) {
 	}
 
 	if x == 0 && y == 0 && width == w && height == h {
+		if pixels != nil {
 			if i.basePixels == nil {
 				i.basePixels = make([]byte, 4*w*h)
 			}
 			copy(i.basePixels, pixels)
+		} else {
+			// If basePixels is nil, the restored pixels are cleared.
+			// See restore() implementation.
+			i.basePixels = nil
+		}
 		i.drawImageHistory = nil
 		i.stale = false
 		return

internal/restorable/images_test.go

@@ -97,6 +97,26 @@ func TestRestore(t *testing.T) {
 	}
 }
 
+func TestRestoreWithoutDraw(t *testing.T) {
+	img0 := NewImage(1024, 1024, false)
+	defer img0.Dispose()
+
+	// If there is no drawing command on img0, img0 is cleared when restored.
+
+	ResolveStaleImages()
+	if err := Restore(); err != nil {
+		t.Fatal(err)
+	}
+
+	for i := 0; i < 1024*1024; i++ {
+		want := color.RGBA{0x00, 0x00, 0x00, 0x00}
+		got := byteSliceToColor(img0.BasePixelsForTesting(), i)
+		if !sameColors(got, want, 0) {
+			t.Errorf("got %v, want %v", got, want)
+		}
+	}
+}
+
 func TestRestoreChain(t *testing.T) {
 	const num = 10
 	imgs := []*Image{}