Player: stop making a List<AABB> every frame.

Shared/Player.cs

@@ -37,6 +37,8 @@ namespace SemiColinGames {
 
     // For passing into Line.Rasterize() during movement updates.
     private List<Point> movePoints = new List<Point>(100);
+    // Possible hitboxes for player <-> obstacles.
+    private List<AABB> candidates = new List<AABB>(10);
 
     public Player(Vector2 position, int facing) {
       this.position = position;
@@ -68,8 +70,7 @@ namespace SemiColinGames {
       residual = new Vector2(movement.X - (int) movement.X, movement.Y - (int) movement.Y);
 
       // Broad test: remove all collision targets nowhere near the player.
-      // TODO: don't allocate a list here.
-      var candidates = new List<AABB>();
+      candidates.Clear();
       // Expand the box in the direction of movement. The center is the midpoint of the line
       // between the player's current position and their desired movement. The width increases by
       // the magnitude of the movement in each direction. We add 1 to each dimension just to be