Customize ffmpeg options when mounting ffmpegfs #155
Description
I use ffmpeg with libopenmpt, which supports -subsong N as a command line option to select a specific subsong from files that contain more than one, or -subsong all to decode/probe all subsongs in sequence. The default, however, is to only read the first subsong, which means that when using ffmpegfs with (e.g.) UMX files that contain multiple subsongs, accessing the transcoded version only gives you the first one.
I don't know if the ffmpeg API used by ffmpegfs makes it possible to specify options like that, but if it does, being able to do so (e.g. -o ffmpeg_opts=subsong=all or similar) would help not just with these files, but with any ffmpegfs usage that requires additional options to ffmpeg to function properly.
For subsong support specifically, I think a more elegant solution would be for each subsong to appear as a separate file, but as far as I know libopenmpt is the only decoder that supports subsongs, so that would likely be significant effort for a very niche use case, while ffmpeg_opts would be more generally useful.