Instead of the somewhat-specific mask-based collision currently used (see https://github.com/cxong/cdogs-sdl/blob/master/src/cdogs/tile.h#L93-L94), refactor the collision system to use collision layers. The code becomes less coupled and allows us to do bullet-bullet collisions; one suggestion was to have mines shootable, another idea is to have gas clouds repel each other.

• Introduce layers for: map objects, bullets, actors; actors on different teams go on different layers
• Refactor actor/ally collision system to use layers
• Move collision code from object/bullet/actor files into collision module