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
Currently proof of concept: Removes wormhole colour from .png texture and specifies it through MT API instead. The post_effect_color may call for registering a separate node per portal colour.
Palette 0 will have to be the traditional portal magenta (instead of 0 being black) for map compatibility with mod when colour was in the texture and not param2.
Goal: make caves a lot brighter, accentuating glowstone and lava
accents, making walking around without torches possible. The old
light level was barely enough to walk and see edges. This level
feels adequate.
Gimp settings: brightness + 40, contrast + 20.
