Large Enemy movement

[igorko]

Migrated from clintbellanger/flare#264

Add functionality to make Large enemies practical, e.g. enemies that require two-tile-wide corridors for movement.

Probably the best way to implement this is to add a collision radius to enemies. A two-tile-wide enemy in the current data might have a radius of 48, so they're unable to pass through a single-tile (64 units) space.

Hazards would need to consider this radius.