We cannot filter this packet purely on the netty thread because our filter
depends on the current camera position (if it's too far, we do want to teleport
it closer) and accessing the camera is only safe from the main thread.
Previously the second part had not been considered which could lead to race
conditions where the camera may not yet exist on the netty thread at time of
handling. These appear to have gotten the wholly inappropriate "just put a null
check around it" treatment, but that means that the filter sometimes doesn't
work (hence the bug report).
This commit changes handling such that all access to `allowMovement` is always
done on the main thread and then moves the filtering there as well. And
therefore filtering should now work properly.
7550e3dcb7