Fix "hide unused cylinder" behavior

There are at least two scenarios where our old code was flat out wrong.
If you manually add a cylinder (because you may want to switch to it in
the profile), then at least until the next time you restart Subsurface
this cylinder should be shown.

Also, when you switch to a cylinder by adding a gas switch event on the
profile, that change should then make that cylinder visible.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>