opusenc: support opus as input

[knarrff]

The documentation currently says "opusenc reads audio data in Wave, AIFF, FLAC, Ogg/FLAC, or raw PCM format" and indeed, when trying to feed it opus, it complains: "Error: unsupported input file".

It would be nice if opusenc would support opus itself as input. This way, e.g., recoding would be much simpler: especially assuming metadata is transferred from the input opus to the output opus. Currently, I don't see another way than to use opusinfo to get the metadata and reinsert that into a pipe from opusdec to opusenc, which is an unnecessarily complex workflow.

[rillian]

opusenc doesn't support lossy input formats because we wanted to encourage encoding with the highest possible quality.