Leaving the Plane of Water to return to a previously visited endgame
level didn't free the air bubbles unless/until you visit a new level.
Returning to that level creates a new set of air bubbles, losing track
of the previous set. Likewise with Plane of Air and its clouds. (Not
an issue with actual save and restore when on those levels, or when
just moving forward to not-yet-visited levels.)
Not applicable to normal play where it isn't possible to return to a
previously visited endgame level.
For 3.7, bubble save/restore ought to become part of savlev() instead
of being handled by savegamestate().
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