Restrict Entity movement to a multiple of UNITS_PER_PIXEL_*

Fixes #968

It's possible that we need to do something similar to this with Hazards.
However, I haven't noticed the bug's behavior with Hazards, so I left
them alone for now.
Merge pull request #1036 from dorkster/fixes

Restrict Entity movement to a multiple of UNITS_PER_PIXEL_*