Fix cancellation of PlayerTrackEntityEvent

[Gerrygames]

Cancelling the PlayerTrackEntityEvent did not remove the player from the seenBy set which leads to updates being broadcast despite the entity not being present on the client and more crucially the entity can only be tracked again by first failing to meet any condition to be tracked.

[lynxplay]

This reads like it'll start spamming this event every time, attempting to track the player on every tick.

[Gerrygames]

Ah yes this is correct. I will add a check if the entity is currently tracked before calling the event.

[Gerrygames]

Hm which will still spam the event when the entity is not tracked. But I think this is how this event is supposed to work.

[lynxplay]

Eh, I think the proper solution is the one we wanted for a while, converting seenBy into a Map that attaches some tracking config.

That way, the entity knows vanilla would track it, but we can prevent e.g. packets being sent. This would also be a massive performance boost to the API canSee/hide/show entity stuff.

[electronicboy]

Yea, as then we can mark it as "tracked" but pretend it isn't, making it easy to allow API to retrack and being able to inline a bunch of stuff much easier rather than several layers of indirection in order to check if we can send to who and what

[Gerrygames]

I do agree that this system in general can be improved. The Entity#canSee check can also be quite heavy when entities are hidden from many players. But I think this is outside the scope of this PR. My goal was to fix that cancelling the PlayerTrackEntityEvent leads to a inconsistent state. If this approach is considered too hard of a performance hit then I am happy to wait for a broader rework of the tracking system. However I don't think this is too much of a problem with my updated implementation. The event will only be called upon initial tracking or for entities for which tracking was cancelled by this event. So for servers that aren't cancelling this event at all there should be no performance impact at all.