Coalesce visual changes to canvas/items and allow canvas to render

Coalesce visual changes to canvas/items and allow canvas to render

First visual change will be processed as normal and then blocked until the
canvas renders the change. If further visual changes need processing then
Editor::pre_render callback will schedule another expose/redraw/render.

This prevents an issue where idle_visual_changer is called many times in
response to events(keys/motion/etc) but the canvas does not get a chance to
render any but the last one which results in a big pause/jump.

This results in a more responsive canvas and in particular a smoother and more
predictable zooming experience.