Index requires sanity check to detect exchanged media file

[LigH-de]

Once an index fle is built, it is assumed to be valid as long as any source file with the original name exists, no matter if the source file has a different content in the meantime, e.g. a video with the same filename but different encoding options was created after the index file was generated.

Comparing if the source file has a newer timestamp than the index file should be a minimal sanity check to rebuild the index file if necessary. Possibly even a checksum of a header portion may be useful in addition.