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/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)