// scrolls the canvas and only a bit of the canvas is invalidated; no
// Clear() is necessary. If FALSE, the whole canvas is invalidated and a
// Clear() is necessary. Disable for when the scroll increment is used to
// scrolls the canvas and only a bit of the canvas is invalidated; no
// Clear() is necessary. If FALSE, the whole canvas is invalidated and a
// Clear() is necessary. Disable for when the scroll increment is used to