image-source: Use mutex when accessing slideshow

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While adding or updating files locked this mutex, the graphics thread
did not. As the update operation is not atomic, the graphics thread
might access the darray in the middle of an update, resulting in access
to freed memory (crash) if the files were updated at the same time.

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500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f