make fields readonly

Shared/Player.cs

@@ -36,9 +36,9 @@ namespace SemiColinGames {
     private float invincibilityTime = 0;
 
     // For passing into Line.Rasterize() during movement updates.
-    private List<Point> movePoints = new List<Point>(100);
+    private readonly List<Point> movePoints = new List<Point>(100);
     // Possible hitboxes for player <-> obstacles.
-    private List<AABB> candidates = new List<AABB>(10);
+    private readonly List<AABB> candidates = new List<AABB>(10);
 
     public Player(Vector2 position, int facing) {
       this.position = position;