Progressively reveal the map to the player

As the character walks around the map, progressively reveal the entities
on the map to them, using an algorithm based on well known
circle-rasterizing and line-rasterizing algorithms to calculate lines of
sight that are potentially obscured by walls.

src/Xanthous/App.hs

@@ -6,6 +6,7 @@ import qualified Brick
 import           Graphics.Vty.Attributes (defAttr)
 import           Graphics.Vty.Input.Events (Event(EvKey))
 import           Control.Monad.State (get)
+import           Control.Monad.State.Class (modify)
 import           Control.Monad.Random (getRandom)
 --------------------------------------------------------------------------------
 import           Xanthous.Command
@@ -60,6 +61,7 @@ startEvent = do
     $ Dimensions 80 80
   entities <>= level
   characterPosition .= charPos
+  modify updateCharacterVision
   -- entities %= EntityMap.insertAt (Position 10 10) (SomeEntity testGormlak)
 
 
@@ -75,7 +77,9 @@ handleCommand Quit = halt
 handleCommand (Move dir) = do
   newPos <- uses characterPosition $ move dir
   collisionAt newPos >>= \case
-    Nothing -> characterPosition .= newPos
+    Nothing -> do
+      characterPosition .= newPos
+      modify updateCharacterVision
     Just Combat -> undefined
     Just Stop -> pure ()
   continue