OTG: Open Terrain Generator Continued

As Open Terrain Generator focuses on development for current versions of Minecraft this project will continue it's development for 1.12.2 specifically.

Features: 

. Modded mob support for biomes

. Disabled pregen for dimensions

. Fixed custom grass colors

. Various performance enhancements

. OTG's 1.16.5 river system backported

Documentation:

Adding modded mobs to biomes

> SpawnCreatures: [{"mob": "oe:crab", "weight": 4, "min": 3, "max": 3}]