Viewport tests

I'm at another good stopping point for these, so here's another pull request. I'm trying to not put them all in one enormous code review, just for sanity's sake.
When you do a `grunt build` it lints the OpenSeadragon code, but it doesn't check for things like that. I'm not sure what one would use to look for things like that.

BTW, if you use Sublime Text, you can install the Sublime Linter plugin, and it'll lint while you type.