gpu/headless: make Screenshot take an input image to tranfer into

When extracting headless.Window's content via screenshots,
it can be useful to keep reusing the same image for output,
as well as specify which area of the Window is to be
extracted.
The updated Screenshot method does this by using the supplied
image.

API change: users must pass an existing image to Window.Screenshot.

Signed-off-by: Pierre Curto <pierre.curto@gmail.com>

gpu/internal/driver/driver.go

@@ -199,17 +199,17 @@ func (f Features) Has(feats Features) bool {
 	return f&feats == feats
 }
 
-func DownloadImage(d Device, t Texture, r image.Rectangle) (*image.RGBA, error) {
-	img := image.NewRGBA(r)
+func DownloadImage(d Device, t Texture, img *image.RGBA) error {
+	r := img.Bounds()
 	if err := t.ReadPixels(r, img.Pix, img.Stride); err != nil {
-		return nil, err
+		return err
 	}
 	if d.Caps().BottomLeftOrigin {
 		// OpenGL origin is in the lower-left corner. Flip the image to
 		// match.
 		flipImageY(r.Dx()*4, r.Dy(), img.Pix)
 	}
-	return img, nil
+	return nil
 }
 
 func flipImageY(stride, height int, pixels []byte) {

gpu/internal/rendertest/bench_test.go

@@ -47,7 +47,8 @@ func resetOps(gtx layout.Context) {
 func finishBenchmark(b *testing.B, w *headless.Window) {
 	b.StopTimer()
 	if *dumpImages {
-		img, err := w.Screenshot()
+		img := image.NewRGBA(image.Rectangle{Max: w.Size()})
+		err := w.Screenshot(img)
 		w.Release()
 		if err != nil {
 			b.Error(err)

gpu/internal/rendertest/util_test.go

@@ -59,7 +59,7 @@ func buildSquares(size int) paint.ImageOp {
 	return paint.NewImageOp(im)
 }
 
-func drawImage(t *testing.T, size int, ops *op.Ops, draw func(o *op.Ops)) (im *image.RGBA, err error) {
+func drawImage(t *testing.T, size int, ops *op.Ops, draw func(o *op.Ops)) (*image.RGBA, error) {
 	sz := image.Point{X: size, Y: size}
 	w := newWindow(t, sz.X, sz.Y)
 	defer w.Release()
@@ -67,7 +67,9 @@ func drawImage(t *testing.T, size int, ops *op.Ops, draw func(o *op.Ops)) (im *i
 	if err := w.Frame(ops); err != nil {
 		return nil, err
 	}
-	return w.Screenshot()
+	img := image.NewRGBA(image.Rectangle{Max: sz})
+	err := w.Screenshot(img)
+	return img, err
 }
 
 func run(t *testing.T, f func(o *op.Ops), c func(r result)) {
@@ -106,7 +108,6 @@ type frameT struct {
 func multiRun(t *testing.T, frames ...frameT) {
 	// draw a few times and check that it is correct each time, to
 	// ensure any caching effects still generate the correct images.
-	var img *image.RGBA
 	var err error
 	sz := image.Point{X: 128, Y: 128}
 	w := newWindow(t, sz.X, sz.Y)
@@ -119,7 +120,8 @@ func multiRun(t *testing.T, frames ...frameT) {
 			t.Errorf("rendering failed: %v", err)
 			continue
 		}
-		img, err = w.Screenshot()
+		img := image.NewRGBA(image.Rectangle{Max: sz})
+		err = w.Screenshot(img)
 		if err != nil {
 			t.Errorf("screenshot failed: %v", err)
 			continue