Fix #21496: RCT1 scenery hidden after reloading

[Gymnasiast]

The problem was caused by the incorrect handling of scenery that was not part of any group, but which attached itself to one. Many scenery ported from RCT1 falls in that category.

Needs testing with some RCT1 and RCT2 scenarios, to see if the behaviour is now correct across the board.

[ZeeMaji]

The fix seems to be working, however in regards to RCT1 scenarios specifically it is unlocking the creepy theming trees in scenarios like leafy lake and bumbly beach when they weren't originally available to the player. But i question if this is worth fixing as it doesn't really hurt to make them available, if anything i ponder if it'd make more sense to make the creepy trees always available in S4 imported scenarios/saves just so that their availability would be more consistent. But that's probably a discussion for another day.

Asides from the creepy trees, everything seems to be working as expected now for the most part. I tested a dozen scenarios ranging from RCT1 scenarios and real parks, RCT2 real parks/expansion parks, along with a giant workbench that had a ton of items that were previously being hidden and they're no longer being hidden now.

However the issue of the misc tab showing items from inventable scenery groups (eg: snow and ice brown wooden fence in Alton Towers) does still exist, that's probably not within the scope of this but i felt like it was worth mentioning as fixing this makes that issue more apparent.