VapourSynth: Providing mismatched `cachefile` causes plugin to error instead of re-indexing

[AndreiVernon]

When FFMS2 is allowed to manage its own cache path, it correctly regenerates the index if the source file no longer matches the existing index. However, when an index path is manually provided via cachefile, FFMS2 instead gives an error The index does not match the source file and does not attempt to regenerate the stale index.

This behavior causes issues for scripts or wrappers that always provide cachefile explicitly (eg. vs-jetpack). In those cases, any modification to the source file requires the user to manually delete the old index file before loading succeeds again.

Desired behavior

If the existing index does not match the source file, FFMS2 should either:

• automatically regenerate the index even when cachefile is manually specified, or
• expose an option to allow automatic regeneration in this case