TODO: Fix start time, relies on correct PTS, currently too late
segment, which is currently ignored as it clears
the subtitle too early.
TODO: Fix end time, normally cleared by a second display
TODO: Implement cropping
TODO: Implement forcing of subtitles
TODO: Handle VC-1 IDUs (Transport level?)
TODO: Decypher remaining bits in extra_data
use direct rendering (no allocation of a new frame)
support a PTS correction mechanism
support more than one output stream