distance changes (until the player crossed a chunk boundary and
triggered the normal update).
This introduces a `ViewDistance` struct that provides an abstraction
around limiting the rate the view distance can be cycled up and down.
This helps avoid unnecessary sending, deleting, and then resending of
synced things like entities (the client will still delete its terrain
locally and re-request it though).
The second part of this fix is storing the last view distance in the
`RegionSubscription` struct and then updating region subscriptions if
this doesn't match the current view distance in the `Presence`
component.
