Allow to change tracking frequency

Hi, this is more of a suggestion. Now that WEBRTC is kind of a thing on mobile devices, I notice that sometimes the track event is a little too processor intense on video from the camera. Mainly because currently the library is relying on requestAnimationFrame to keep the tracking live. Any chance the tracking frequency could be passed as property to the constructor?

Start by locating the requestAnimationFrame loop and the constructor option handling. Define how a tracking-frequency property should control camera-video tracking, then verify that tracking remains live while processor work is reduced.

Written by the indexing model from the issue text.