Slight lag reduction

Added some code to PolygonGenerator to destroy extant water objects and
clear current mesh when generating new terrain to increase the
difficulty.
Current lag status:
1. No lag upon starting new game.
2. No lag upon increasing difficulty once.
3. No lag upon increasing difficulty twice.
4. Brief lag upon increasing difficulty third time, lag goes away after
a few seconds.
5. Fairly consistent lag upon increasing difficulty fourth time. Might
be related to undeleted objects clogging up the game, or might be
related to all the water objects. Unsure. Will do more testing.