make NPCs stop at the edge of platforms

Shared/NPC.cs

@@ -26,8 +26,9 @@ namespace SemiColinGames {
     public string Update(NPC npc, float modelTime, World world) {
       int moveSpeed = 120;
       int desiredX = npc.Position.X + (int) (moveSpeed * npc.Facing * modelTime);
+      int testPoint = desiredX + 12 * npc.Facing;
       // TODO: define the box modularly & correctly.
-      AABB npcBox = new AABB(new Vector2(desiredX, npc.Position.Y), new Vector2(1, 33));
+      AABB npcBox = new AABB(new Vector2(testPoint, npc.Position.Y), new Vector2(1, 33));
       Debug.AddRect(npcBox, Color.Cyan);
       bool foundBox = false;
       foreach (AABB box in world.CollisionTargets) {