app/headless,gpu/gl: make ReadPixels y-flipping backend specific

The Direct3D backend doesn't need y-flipping, so don't do it unconditionally in
package app/headless.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/headless/headless.go

@@ -127,25 +127,9 @@ func (w *Window) Screenshot() (*image.RGBA, error) {
 	if err != nil {
 		return nil, err
 	}
-	flipImageY(img)
 	return img, nil
 }
 
-func flipImageY(img *image.RGBA) {
-	// Flip image in y-direction. OpenGL's origin is in the lower
-	// left corner.
-	row := make([]uint8, img.Stride)
-	sy := img.Bounds().Dy()
-	for y := 0; y < sy/2; y++ {
-		y1 := sy - y - 1
-		dest := img.PixOffset(0, y1)
-		src := img.PixOffset(0, y)
-		copy(row, img.Pix[dest:])
-		copy(img.Pix[dest:], img.Pix[src:src+len(row)])
-		copy(img.Pix[src:], row)
-	}
-}
-
 func contextDo(ctx context, f func() error) error {
 	errCh := make(chan error)
 	go func() {