This commit mostly removes unused code and cleans some things up.
The main contribution is that instead of calling repaint() and Refresh(), we
now only mark the canvas as dirty and call Refresh(), which will do the repaint
just-in-time. This speeds things a lot when quickly changing layers or zooming,
as it does not repaint once per keyboard/mouse event but once per actual
Refresh, which makes it much smoother.
db04449a2b