Add a way to set scale for Camera Tracker

[Jaydude-wb]

The Camera Tracker seems to guess the scale of a scene, resulting in enormous spaces without any relation to the shot footage. It would be nice to have an approximation of the real world scale.

This can be done in Cinema 4d style, by defining a 'known length' between two tracked points.

Please add something alike.

[Mylenium]

Yeah, definitely. The tracker could have needed calibrated references from day one.

Mylenium