Multiple canvases with SDL and Emscripten

### multiple canvases
I tried to create a game which requires multiple canvases, the problem is that there is no way to control more than 1 canvas. I request a feature so that you can **address multiple canvases at once from a single c++ file from SDL**.

### SDL to HTML5 Canvas
As a side note it would be great if you could **release a documentation for the SDL to html canvas conversion** in the generated Javascript file, so that the javascript in the project can be minimized to the simplest form.