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/Game/Lenses.hs

@@ -5,6 +5,7 @@ module Xanthous.Game.Lenses
   , character
   , characterPosition
   , updateCharacterVision
+  , characterVisiblePositions
   , getInitialState
   , initialStateFromSeed
 
@@ -84,12 +85,16 @@ characterPosition = positionedCharacter . position
 visionRadius :: Word
 visionRadius = 12 -- TODO make this dynamic
 
--- | Update the revealed entities at the character's position based on their vision
+-- | Update the revealed entities at the character's position based on their
+-- vision
 updateCharacterVision :: GameState -> GameState
-updateCharacterVision game =
+updateCharacterVision game
+  = game & revealedPositions <>~ characterVisiblePositions game
+
+characterVisiblePositions :: GameState -> Set Position
+characterVisiblePositions game =
   let charPos = game ^. characterPosition
-      visible = visiblePositions charPos visionRadius $ game ^. entities
-  in game & revealedPositions <>~ visible
+  in visiblePositions charPos visionRadius $ game ^. entities
 
 data Collision
   = Stop