#5 ink redrawing is slow when zooming

[Kevin Conroy]

due to a hack to prevent "wandering/ghost" ink when a
person scrolls or updates a document, we end up
redrawing strokes two to three times.

make a queue, a cache, anything to improve this redraw
performance as it hinders usability when working with a
larger document with many strokes (particularly many
overlapping large strokes)