dashenc: default to one AdaptationSet per stream

Previously all mapped streams of a media type (video, audio) where assigned
to a single AdaptationSet. Using the DASH live profile it is mandatory, that
the segments of all representations are aligned, which is currently not
enforced. This leads to problems when using video streams with different
key frame intervals. So to play safe, default to one AdaptationSet per stream,
unless overwritten by explicit assignment.

To get the old assignment scheme, use

  -adaptation_sets "id=0,streams=v id=1,streams=a"

Signed-off-by: Peter Große <pegro@friiks.de>
Signed-off-by: Martin Storsjö <martin@martin.st>