Don't render moving entities that aren't visible

When the character walks away from or around the corner from entities
that move such that they're no longer visible, stop rendering them.
Still render static entities like walls, doors, and items though. This
prevents entities walking into a "revealed position" after the
character's left being visible despite not being in a line of sight any
more.

src/Xanthous/AI/Gormlak.hs

@@ -90,10 +90,13 @@ newtype GormlakBrain = GormlakBrain Creature
 
 instance Brain GormlakBrain where
   step ticks = fmap coerce . stepGormlak ticks . coerce
+  entityCanMove = const True
 
 --------------------------------------------------------------------------------
 
-instance Brain Creature where step = brainVia GormlakBrain
+instance Brain Creature where
+  step = brainVia GormlakBrain
+  entityCanMove = const True
 
 instance Entity Creature where
   blocksVision _ = False