FLAC compression levels instead of "output quality"

[rmeissn]

In the current version, the FLAC output quality is limited to "auto" and "16 bit" settings, without an option for adjusting the compression level. In contrast, when using vorbis or mp3 formats, the output quality directly affects both the sound quality and the file size, making it the most commonly used parameter. However, with FLAC, the most commonly adjusted parameter is the compression level, which directly impacts the file size, as FLAC is a lossless codec. Therefore, using 16 bit as a quality setting in FLAC is akin to downsampling the audio, which isn't provided as an option for vorbis or mp3. It would be more convenient to have a compression level option for FLAC, rather than being limited to the output quality settings of "auto" or "16 bit".

See https://ffmpeg.org/ffmpeg-codecs.html#flac-2