(new animation by Extex101)
The original texture is now "nether_portal_alt.png", and used to provide a "nether:portal_alt" node, which is used/demoed by the Surface-portal as the original animation still has a "it's full of stars" charm coming out when used for lighter coloured portals.
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
If a portal is being ignited and portal frame near-but-not-at its destination was found that was active and already linked back to the local portal, it would cause the local portal to be extinguished. (Required the remote portal to somehow be in a different state from the portal it was linking to)
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.
Also adds nether.register_wormhole_node(), moving the wormhole node template into portal_api.lua, and allowing wormhole nodes with custom post_effect_color to be more easily created.
When a surface level was known, only the volume_is_natural check was being performed, this fixes that.
Also fixes bug where LBM didn't start the timer of example portals that had been disabled.
Allows portal ignite to repair a stopped timer.
Other misc cleanup - paramat's new Floatlands won't have lakes so I removed that from the flavortext.
give nether_portal_circular.mts a flat floor - the new nether_portal_circular.mts allows its bottom nodes to be sunk into the ground.
Example portal basic behavior and book_of_portals_pagetext implemented.
Other changes are allowing portals corrupted by ABMs to be repaired, finding better ground level of surface portals.
UTF-8 with BOM crashes some systems according to 6551f5c120 comment, and there are several cases where Minetest is buggy with files that use CRLF line endings (though none I'm aware that affect these files), so strip CRs and BOM
This requires a cache-invalidation hack borrowed from cloudlands mod.
Also, null reference fixes in ensure_remote_portal_then_teleport and locate_or_build_portal, and minor improvements
Fix bug where ensure_remote_portal_then_teleport() converted its destination_wormholePos to an anchorPos to pass to locate_or_build_portal() which converted it back to an wormholePos. Bug occurred when local_orientation was assumed for the target orientation, which could lead to locate_or_build_portal() ending up with a reconstituted wormholePos rotated outside the portal frame, not find a portal frame, so build a new portal there, griefing the old one.
locate_or_build_portal() has been refactored to take a wormholePos instead of an anchorPos, removing the need to know the orientation of the target portal (which can't be obtained from the param2 when the target portal is unlit)
This is effectively a missing part to the change that made locate_or_build_portal() enforce that portals only link together in mutual pairs. As target portals that didn't need to be located or built we not being updated to link back to where the player travelled from.
Also
* implements events: on_run_wormhole, on_extinguish
* adds a lot of debug info
