Add function to compute size of displayed videos without letter- or pill...

...arboxes.

First chunk of <https://trello.com/c/d6UTAWN1/243-as-a-user-i-should-always-see-the-local-video-stapled-to-the-remote-video-with-no-unnecessary-letterboxing-so-things-look-reason>, separated out for ease of review.

While it's currently only going to be used in the CallView, it was utterly unrelated to the object logic itself, and may be generally useful elsewhere, so I put it in app.utils instead as a public API (poor man's version of the humble object pattern!)

A few bits are untested / unimplemented, as I'm not sure what the ultimate API wants to be.  I expect that will become clear and I'll test/implement those as part of the rest of the card.