Move tracker construction into ITMTrackerFactory.

The intention is to facilitate reuse when InfiniTAM is used as an external
library. The change also has the side-benefit of making the code in
ITMMainEngine slightly shorter.
Move scene from ITMTrackingState -> ITMRenTracker.

ITMTrackingState isn't templated, so we can't store a scene pointer in it
without hard-coding the template parameters. This prevents external code
from using custom voxel types. By moving the scene pointer into
ITMRenTracker, which is both templated and the only place in which the
pointer is used, we solve the problem.
fixed a segfault in freeview visualisation

- used to occur after the end of an input image sequence only