Implemented destructible terrain.  Integrated into 'Arena' and 'Rect2'.  To add destructible terrain into an arena, add a Rect2 with the keyword arg 'hits_to_destroy' with a positive int value when creating the Arena object.  See current 'arena1' object for example.  Destructible terrain currently not handling hits from 'MeleeParticle's correctly (too many hits taken).  Will work on fixing this next.