Better sync between drawing canvas & video size

Not clear why center is relative to video dimensions but canvas size is
relative to the other canvas (which draw already existing annotations).

There may be some bugs from paperjs, see the following issues
https://github.com/paperjs/paper.js/issues/662
https://github.com/paperjs/paper.js/issues/668
https://github.com/paperjs/paper.js/issues/1757