Fix for major graphical issue, smaller tweaks.

World.rendervisibletiles() finally made to work with layering.
Player.updatevisible() rewritten for clarity (using same algorithm).
Bears send a message and scatter some player coins on contact.
Tidier coordinate manipulation.
Reduce 0.05s loop delay by moving/rendering time to reduce lagginess.
Replace all docstring ''' with """ per PEP 257 (KDevelop is happy now).
Some other tweaks.