Simple MapEntity visibility culling (structures outside the camera view or items with nothing to render aren't drawn)

Could be optimized further, but already gives a pretty nice performance boost

Subsurface/Source/Map/MapEntity.cs

@@ -319,6 +319,11 @@ namespace Barotrauma
             mapEntityList.Insert(i, this);
         }
 
+        public virtual bool IsVisible(Rectangle worldView)
+        {
+            return true;
+        }
+
         public virtual void Draw(SpriteBatch spriteBatch, bool editing, bool back=true) {}
 
         public virtual void DrawDamage(SpriteBatch spriteBatch, Effect damageEffect) {}