API improvements

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

settingtypes.txt

@@ -15,5 +15,5 @@ nether_realm_enabled (Enable Nether realm & portal) bool true
 # Enables the Floatlands portal api code example
 nether_enable_portal_example_floatlands (Enable example portal: Floatlands) bool false
 
-# Enables the Surface travel portal api code example
-nether_enable_portal_example_surfacetravel (Enable example portal: Surface travel) bool false
+# Enables the Surface-travel portal api code example
+nether_enable_portal_example_surfacetravel (Enable example portal: Surface-travel) bool false