vis.js: help lazy garbage collection on canvases

on machines with low amount of graphics memory lazy garbage collection will cause mismatch between what browser think it has on screen and what graphics driver show on screen.

Canvas reserved graphics memory is not released until garbage collection happen. Here is set canvas size to zero before it is discarded, this seem to help to ease memory issues on mobile devices.

Signed-off-by: Juha-Pekka Heikkila juhapekka.heikkila@gmail.com