More robust pose tracking

[puzzlepaint]

Given that pairwise frame tracking seems to be very fast, in the odometry part, one could try to always track wrt. multiple previous keyframes (separately) instead of only wrt. the latest keyframe. This would allow to average the results, and also enable detecting tracking issues (when the results of tracking wrt. different keyframes differ too much).

This perhaps only applies to the case of not using an IMU, since using an IMU might robustify the local tracking to the point where such measures would not really help anymore.