- Validate input arguments: for example some ints must be >= 0
- Add options to control the delay of when frames are added or removed from
  the trail
- Some trivial cases of trails just need a framebuffer with no need to keep
  the history of frames, see if the code handling those cases can be optimized
- Clean up the code? My C++ is a little C-ish...
- Evaluate other background subtraction methods:
  https://github.com/opencv/opencv_contrib/blob/master/modules/bgsegm/samples/evaluation.py