Fixes https://trac.ffmpeg.org/ticket/2798

This makes movenc handle AV_DISPOSITION_ATTACHED_PIC and write
the associated pictures in iTunes cover atom. This corresponds
to how 'mov' demuxer parses and exposes the cover images when
reading.

Most of the existing track handling loops properly ignore
these 'virtual streams' as MOVTrack->entry is never incremented
for them. However, additional tests are added as needed to ignore
them.

Tested to produce valid output with:
  ffmpeg -i movie.mp4 -i thumb.jpg -disposition:v:1 attached_pic \
         -map 0 -map 1 -c copy movie-with-cover.mp4

The cover image is also copied correctly with:
  ffmpeg -i movie-with-cover.mp4 -map 0 -c copy out.mp4

AtomicParseley says that the attached_pic stream is properly
not visible in the main tracks of the file.
Signed-off-by: Timo Teräs <timo.teras@iki.fi>