Implement serverside entity culling

[TestingPlant]

For each player, if another player is not visible to them (such as being hidden behind a wall), the other player should be despawned. This has some advantages:

• Decreases the number of position/look packets sent to the client to reduce network utilization
• Improves client performance

The second point is the most important. I have tested on both Fabric with Sodium and Optifine, and both are able to get >60 fps with no players spawned. However, with 2k players spawned, the performance drops substantially, even when the players are not in view.

On Sodium:

On Optifine:

The event and the event map should be designed in a way that ~<1k players are usually visible at the same time.

For the implementation, I'm thinking of the following:

• Divide the world into 4x4x4 chunks
• Have a separate program which pregenerates a list of chunks that are visible at a particular chunk given the map. This list will be used to make a Bloom filter or similar data structure
• To determine whether a chunk is visible to a specific chunk, we will do raycasts from many points within the chunk. This is expensive which is why it's being pregenerated
• On the proxy, we'll have channels which the server can subscribe and unsubscribe players to. A player's position and look updates correspond to one channel. Using the chunk visibility information, the server will subscribe and unsubscribe visible and hidden players accordingly

Advantages:

• This should be relatively fast. Bloom filters can check whether an item may be in the set in constant time. We may want to also use a different probabilistic data structure instead of Bloom filters; there are many newer ones that claim to be faster than Bloom filters.
• We could implement the visibility checks entirely on the proxy and do nothing on the server

Disadvantages:

• Requires map blocks to be unbreakable (we are likely to do this anyways)
• Does not account for blocks placed at runtime, although this should not be a major issue