Prevents the c++/native mapgen from placing ores or decorations (when useBiomes is true).
The biomes-based implementation is faster, more deterministic, and keeps its dungeons, but requires MT 5.1 features to work.
Also
* Glowstone & Netherrack stalactites
* Include the new decorations in mapgen_nobiomes
* Decorate dungeons - Add windows and glowstone "chandeliers" to some rooms
* Configurable Nether floor and ceiling depths
Improvements found while using the portals API in another mod (cloudlands):
* remote_portal_checkup() will check the whole portal and frame for mapgen overwrites.
* Allow find_realm_anchorPos() to return nil if no realm location could be found (portal will fail to ignite).
* Allow create_book_of_portals() to be invoked [indirectly] by other mods.
Also lowers nether_book_close.png to be 4bpp
https://forum.minetest.net/viewtopic.php?t=15912
Don't place Book of Portals in dungeon chests if its contents can be written in the help modpack Encyclopedia instead, and don't add Book of Portals to dungeon chests if Nether portals are the only type of portal - A Nether mod doesn't need a Book of Portals if it's only being used to provide a Nether portal.
