Fix issue switching between scenes with many entities

[netpro2k]

When switching between scenes with > 100 entities (which is not even all that many) BitECS's entity id recycling causes some strange behavior in our object3d cleanup code.

This PR (along with a patch to BitECS) allows entity id recycling to be configured, and configures it to 20% of our 10,000 entity max (2000). This points at our own fork of BitECS for now until the change lands upstream. This is still only a partial solution, as the issue would still happen if entities are added on a frame where > 2000 entities were also removed, but this is very unlikely to happen.

There was some discussion of further solutions on github and discord

[takahirox]

Curious to know, any downside of choosing 20% that is much greater than the default 1%? The recycling process time can be longer?

[netpro2k]

There is no cost to the "recycling" itself, its just a difference in addEntity between an id coming from the removed ids array or the next incremental entity id (https://github.com/NateTheGreatt/bitECS/pull/93/files#diff-4833659b31bb551bd2b6b1f136dee5686fcbcc1c29fb9de498950066d591eb62R87). The "cost" of not recycling is sooner is that you get a bit more memory fragmentation in your component data arrays.

[takahirox]

Thanks, probably better memory fragmentation may not be our first priority then 20% is acceptable for us.