Detect tracking failures, build multiple models, and re-localize

[puzzlepaint]

Currently, the program will not detect when it fails to track the camera pose, giving wrong results. It would be good if it could determine failure and state something like "tracking lost" instead.

If tracking continues after a failure, the program should start building a new model, while trying to merge the new model with the old one by re-localizing against it, if possible.