app/headless: compensate for OpenGL's lower left origin

The origin of image.Images is the upper left corner. Convert the
ReadPixels result by flipping the image the y-direction.

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

app/headless/headless.go

@@ -111,6 +111,17 @@ func (w *Window) Screenshot() (*image.RGBA, error) {
 		}
 		return nil
 	})
+	// Flip image in y-direction. OpenGL's origin is in the lower
+	// left corner.
+	row := make([]uint8, img.Stride)
+	for y := 0; y < w.size.Y/2; y++ {
+		y1 := w.size.Y - 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)
+	}
 	return img, nil
 }