Consider caching interested entities

[HeroesGrave]

I'm not sure if performance is that much of a concern, but I thought I'd throw this in anyway. Feel free to close this issue if it's not going to be a problem.

Currently you have to filter through all entities every update cycle to get those a system is interested in. A system cannot (or should not?) lose interest in an entity unless its components are added to or removed from.

Therefore, you could get a nice performance increase by only checking modified entities, and caching interested entities with each system.

[johann2]

I don't know, bitmasks should be really fast, especially if you do it over a vector. And since caching adds significant complexity, I'll first write some benchmarks to see if it's a problem at all.