increment before assignment

[yrrah]

Please let me know if this is correct:

The background / unlabeled part of a mask is index 0.

If you have two objects being tracked, they would be index 1 & 2 and
SegTracker.curr_idx = 2

Then when SegTracker.find_new_objs() finds one additional object, it should set the index in the resulting combined mask to SegTracker.curr_idx + 1 = 3

Currently the increment happens after assignment, so the third object incorrectly gets assigned SegTracker.curr_idx = 2

This might be the cause of #105