Porting to Unity VIII: Biome/rivers autotiling

Another straightforward port, based on the posts:

• /sigil-of-kings-website/2018/02/09/autotiling-adventures/
• /sigil-of-kings-website/2018/02/23/autotiling-adventures-part-ii-procedural-masks-for-biomes-and-rivers/

Captured video, including sprites: