HLS protocol version 4 added alternative renditions to the
specification (e.g. alternative audio tracks).

The EXT-X-MEDIA tags can also contain metadata for "renditions" (i.e.
tracks) of the main Media Playlist.

Add support for those.

Note that the same rendition (AVStream) may be associated with multiple
variants (AVPrograms).

Alternative subtitle tracks will require additional work and are
therefore not enabled yet.
Signed-off-by: Anssi Hannula <anssi.hannula@iki.fi>