entities are synced from and displayed in.
NOTE: Syncing entities work at the granularity regions which are
multi-chunk squares but the display of entities in voxygen is limited in
a circle with the radiues of the supplied distance.
Additional details and changes:
* Added `ViewDistances` struct in `common` that contains separate
terrain and entity view distances (the entity view distance will be
clamped by the terrain view distance in uses of this).
* View distance requests from the client to the server now use this
type.
* When requesting the character or spectate state the client now passes
its desired view distances. This is exposed as a new parameter on
`Client::request_character`/`Client::request_spectate`. And the client
no longer needs to send a view distance request after entering these
states. This also allows us to avoid needing to initialize `Presence`
with a default view distance value on the server.
* Removed `DerefFlaggedStorage` from `Presence` and `RegionSubscription` since the
change tracking isn't used for these components.
* Add sliders in voxygen graphics and network tabs for this new setting.
Show the clamped value as well as the selected value next to the
slider.
* Rename existing "Entities View Distance" slider (which AFAIK controls
the distance at which different LOD levels apply to figures) to
"Entities Detail Distance" so we can use the former name for this new
slider.
