Feature/pt hysteresis

Enables per-pixel hysteresis for the point extractor, can be disabled in configuration GUI (new slider under threshold). 

Drawbacks: somewhat higher CPU demand (no idea how much or little it really is), point locations can be a little distorted 
Benefits: immunity to camera noise while keeping full responsiveness to both sudden, fast head movement and slow, unidirectional head movement
Side effect: tiny oscillations of the head are lost together with the pixel noise, this might be good or bad depending on use case

I implemented the same for my personal build of Freetrack several years (and many happy hours of use) ago, so i consider the general principle very useful.

Enjoy, Ulf

PS: oh, the fork info at the end of the readme made it into the pull req, you probably want to exclude that...