Add option to run incremental renders on sync thread, using same chunk

logic as timesliced full render, in order to handle cases where
assumption that all chunks needed for incremental render are already
resident (which can be false if player telepots, move quickly,
multiple players are active, etc).

configuration.txt

@@ -6,6 +6,9 @@ display-whitelist: false
 # How often a tile gets rendered (in seconds).
 renderinterval: 1
 
+# Do render on main thread - may generate more server load, but safer and fixes broken tiles
+renderonsync: true
+
 render-triggers:
 #  - playermove
 #  - playerjoin