Call av_metadata_conv() to convert metadata to ffmpeg's generic format

This is actually needed for everything to work properly, but it only really
started breaking with newer versions of ffmpeg where more demuxers have been
completely ported over to the metadata API.

src/filescanner_ffmpeg.c

@@ -452,6 +452,8 @@ scan_metadata_ffmpeg(char *file, struct media_file_info *mfi)
       goto skip_extract;
     }
 
+  av_metadata_conv(ctx, NULL, ctx->iformat->metadata_conv);
+
   if (ctx->metadata)
     {
       ret = extract_metadata(mfi, ctx->metadata, extra_md_map);