stream_dvda: expose DVD-Audio still images as a cover-art video track

DVD-Audio discs carry an Audio Still Video Set (ASVS). An MPEG-2 still
images (cover art, booklet/lyric pages) that the disc associates with
tracks.

A new STREAM_CTRL_GET_STILL returns the still for a given playback time.
demux_disc exposes it as a selectable mpeg2video track and injects the
still as a keyframe at the track's playback time. It changes as playback
crosses track boundaries (and follows onset times when a disc encodes
them).

--dvda-page (with the new STREAM_CTRL_SET_STILL_PAGE) forces a specific
still page instead of the scheduled one. The still state is guarded by
a mutex, runtime page changes arrive from the player thread while the
demuxer thread extracts stills.

DOCS/interface-changes/dvda.txt

@@ -1,2 +1,2 @@
 add `dvda://` protocol for DVD-Audio playback
-add `--dvda-device` option
+add `--dvda-device` and `--dvda-page` options