Since decals may have thinkers attached this will crash when such a savegame gets loaded, because the thinker lists get reset in P_SerializeThinkers, deleting any thinker that already was processed.
I also added an error message that immediately aborts the save process if such an out-of-sequence thinker is attempted to be written out.
This obviously breaks savegame compatibility again...
* set up linked sector portals so that everything that will eventually have to be considered is present, even though the software renderer currently can't handle those adequately.
* tag all skybox things with a type so that they can easily be distinguished at run time.
* fill in the linked portal types in xlat/eternity.txt.
- fixed: Sector_SetPortal's Eternity translation was not correct.
The ZDoom variant allows specifying the plane as part of the portal, Eternity does not. Added a new 'plane' type 3 which just means 'any'.
