1. Consider 'own_world' as well as 'world' to stop propagating enter/exit world notifications.
2. Clean & fix handling of camera currency.
This fixes some random crashes and error logs in the editor; namely
- when enabling/disabling own world in a Viewport;
- when switching back from a subscene displayed into a main scene's Viewport;
- when exiting the editor after any of them;
- memory corruption (can that explain certain other seemingly unrelated crash reports?).
This also fixes situations where a Viewport and its main Camera get out of sync about which World is relevant to them.
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