Cache file very big due to album art metadata when using ogginfo

Hi,

In the last few months (not sure since when exactly) I've noticed the my
EMMS cache is taking a very long time to load.  A few days ago I had
some free time to investigate and I realized that my cache file was over
4 GB in size (with around 1000 tracks).  Further investigation revealed
that for each track a value called 'info-metadata_block_picture' was
saved, which I'm assuming is the embedded album art encoded in some way.
Removing this value (by adding a function to 'emms-info-functions' which
sets it to nil) shrunk my cache to under 3 MB.

AFAICT, this value isn't used by EMMS, so maybe it shouldn't be saved to
the cache in the first place?

Regards,
Daniel

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