adaptivedemux: Enable bitrate selection for trick mode streaming again

adaptivedemux: Enable bitrate selection for trick mode streaming again

And scale the bitrate with the absolute rate (if it's bigger than 1.0) to get
to the real bitrate due to faster playback.

This allowed in my tests to play a stream with 10x speed without buffering as
the lowest bitrate is chosen, instead of staying/selecting the highest bitrate
and then buffering all the time.

It was previously disabled for not very well specified reasons, which seem to
be not valid anymore nowadays.