Add support for mad (high-quality mp3 decoder library and cli frontend)

Packages describing “mad” as local USE flag

Package “mad” Flag Description
media-libs/xine-lib Build the media-libs/libmad based mp3 decoder. This mp3 decoder has superior support compared to the one coming from FFmpeg that is used as a fallback. If you experience any bad behaviour with mp3 files (skipping, distorted sound) make sure you enabled this USE flag.
sys-fs/mtpfs Enable handling of MP3's metadata

All packages providing a “mad” USE flag (21)