if the request/cancel/re-request happens all in the time before the queue gets drained, chunk hashes may appear multiple times in the queue. We don't want to process them twice if this happens (although it's mostly harmless anyway).
this can happen especially on large render distances when flying fast and changing direction - we decide we don't want the chunk, then, after changing direction and re-ordering chunks, we decide we do want it again, and end up registering a second callback. In this case, we need to ensure that only one of the callbacks gets executed (it doesn't matter which one).
this could happen if a plugin calls setPopulated(true) on a chunk after a request for its population landed in the queue, but before it actually got processed. In that case, the promise would never get fulfilled.
this does not indicate a failure; it indicates that the chunk has already been successfully populated.
In this case, we shouldn't be putting the task back on the queue.
This is skirting around the real bug, which is that requestChunkPopulation() doesn't check if the target chunk is already populated before it creates a new promise that it will be.
This reverts commit 866020dfdb.
For some fucking reason this broke resending chunks in some cases (and
sending chunks at all in others). I don't have time to debug this right
now, so it's going to have to remain broken, infuriatingly enough.
this allows chunks locked for population to be modified. If the PopulationTask detects that the chunk was modified during the onCompletion(), the result of the population will be discarded and rescheduled, so that it includes user modifications.
Player->showPlayer() assumes that spawnTo() will take care of all the checks necessary to ensure we don't actually spawn a player to someone it shouldn't be able to see. In PM3, there's nothing we can do about that.
This could be a problem if anything decides to override spawnTo() to do additional stuff and assumes that the function will always succeed; however, there's not much reason to do that (plugins sending packets should override sendSpawnPacket() instead).
this should never be hit in the PM case, but it's an InvalidArgument rather than AssumptionFailedError because plugins can and do call this with bad things.
to be consistent with Position::__construct() and Location::fromObject() (although Location::fromObject() has no choice, thanks to the anti-feature known as late static binding ...)
I did consider allowing yaw/pitch to remain optional, but considering the implicit immutability of Location, it really doesn't make any sense to create a Location with default yaw/pitch - just create a Position in that case instead.
