Fix aspect ratio on video so that there is no black space

[simonguest]

[mfripp]

I'd really appreciate the option to crop rather than letterbox when resizing. Then I could make a square or vertical floating window that just shows me, instead of a widescreen view with unneeded background. This could also be an easy alternative to freezing the aspect ratio, since it would also eliminate the letterboxes and users could naturally choose the shape that is right for them.

This might be as simple as adding self.captureLayer.videoGravity = AVLayerVideoGravity.resizeAspectFill;.