Make the MultiCanvas one canvas element in the DOM structure?

[martinRenou]

Instead of stacking each Canvas view on top of each other to build the MultiCanvas in the DOM, we might want to use one canvas element on which we draw all layers from the CanvasModels.

This would have some benefits:

• The DOM structure will be simpler
• The MultiCanvas sizing/layout will behave exactly like a normal Canvas
• We don't need to create multiple Canvas views, we can use offscreen canvases

We might loose some performances though. As when one canvas updates, we need to redraw all layers using drawImage.