// bizarrely, we only need to scroll on the x value here as fastpaint
// copies the full height of the image anyway. Passing in the y value
// causes nasty repaint artefacts, which only disappear on a full
// bizarrely, we only need to scroll on the x value here as fastpaint
// copies the full height of the image anyway. Passing in the y value
// causes nasty repaint artefacts, which only disappear on a full