Add Triangle Threshold algorithm after upgrade to OpenCV 3

[adius]

#315 already adds otsu's algorithm.

It's just necessary to add another else branch:

else if (strcmp(*algorithm, "Triangle") == 0) {
  typ += 16;
}

[peterbraden]

Can we wrap this in a macro so it remains backwards compatible?

[adius]

Can you give an example? I'm not sure what you mean by macro.