Loader LoadState processing should avoid redundant work

[kabergstrom]

Currently the main LoadState state machine iterates through all LoadStates every frame. Generally it only needs to iterate over states that may change. I think avoiding processing of Loaded assets will be sufficient, and can probably be achieved by processing only when a ref is added or removed.

[kabergstrom]

It would also be nice to progress the LoadState state machine multiple times per frame to reduce loading latency. Maybe run the process function until the state doesn't change?