[Bug]: Story mode: disappearing boats/granitos

[STEMathematics]

SuperTux Version

supertux2 dev v0.7.0-beta.2 - 9644491 (master)

System Information

Linux Mint 22.3 - Cinnamon 64-bit

Expected Behavior

Boats and granitos do not disappear from the worldmap.

Actual Behavior

If the world is switched away from while Tux is on a boat or granitos, that disappears when Tux switches back to the world.

klip_1.771.537.150.875.mp4

Steps To Reproduce Actual Behavior

1. One of the following:
   a) Get onto the boat at the end of Icy Island.
   b) Get onto the boat at the start Rooted Forest.
   c) Get onto the granitos at the end of Rooted Forest.
2. While on water, switch to another world.
3. Switch back.
4. Notice the boat/granitos Tux was on disappeared.

Additional Information

No response

Guidelines For Reporting Issues

• I have read https://github.com/SuperTux/supertux/blob/master/CONTRIBUTING.md#bug-reports.
• I have verified this isn't an issue that's already been reported.
• I have verified this isn't a discussion, or an issue about a crash or a feature request, but rather an actual bug ─ that is, the game did something not intended.
• I have verified this issue is not about wrong translations (use Transifex for those), or anything unsupported (e.g. third-party add-ons).
• In this report, I have only included details about one (1) bug.
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