Remove the canvases cache

The cache has been added in #3312 in 2013 and a lot of things changed since.
Having too many cached accelerated canvases can lead to have to move their data from the GPU to the RAM
which is costly.
So this patch:
 - removes all the cached canvases;
 - destroys the useless canvases in order to free their associated memory asap;
 - slightly rewrite canvas.js::_scaleImage to avoid too much canvas creation.

Author: calixteman