medialibrary: Thumbnailer: fix double-free and add new test
The picture was referenced on the ThumbnailerCtx object, which release it whenever one is not nullptr. However, after the changes in !3088 (merged), commit 2ea2c7eb, the release policy for the picture being exported has been fixed to ensure the picture is consumed whatever the conversion case.
Ensure the picture won't be released twice by swapping it from the ThumbnailerCtx object before exporting.
Fix #27749 (closed)