Performance optimizations

Minor performance improvement patch to address the extreme lag which occurs after accumulating a lot of XP orbs on the map.

This patch does the following:

• All loot items farther away from the player will now get disabled. This will also, though unintentionally, nerf the Global Orb Magnet (previously it attracted all XP orbs from anywhere on the map. Now it won't do that to orbs too far away from the player).
• The Spine-Thorn enemy will now also get disabled the same way.

Hopefully, this will improve the lag a lot in the late-game!