Make obsidian cost 3 ems for 2 obsidian not 3

Currently you need 6 ems to get obi cuz 3ems = 3 obsidian, but that is way to ez to speedrun, with the rate at which emeralds spawn you can bridge to mid, go thru each em gen and leave then get insta obi, with this added you now need 9 ems to get it, meaning even if the rate is still fast you have to get the max of 4 ems in each gen plus another one.