support all display modes

Fixes #1 by adding `getDisplayMode` to `DeckLinkController`, which will return the correct `BMDDisplayMode` mode based on width, height and framerate args.

`framerate` in `ofxBlackMagic::setup` changes to `int` to match `getDisplayMode`, this shouldn't break existing code (?), but means we can be certain about comparing given frame rate values. 

Because of truncation we can still accept a `float` (e.g. `29.97` will become `29`). When passing an `int` they can either be two digit (e.g. `29`, `30`) or 4 digit (`2997`, `3000`).