[opus] [user] coverart and other tags

[Timothy B. Terriberry]

Alice Wonder wrote:
> I found a solution. If I add the metadata_block_picture to the flac file
> I use as input, opusenc copies it over and at least in totem (linux
> movie player) it displays when playing the Opus file.

You can also add it to the Opus file directly with the --picture 
argument to opusenc (documented via opusenc --help or on the man page). 
This argument works the same way it does with FLAC.