videodecoder: Detect buffers inputted with DTS

videodecoder: Detect buffers inputted with DTS

Some container formats (like AVI) set DTS on the buffers instead of
PTS.
We detect this by:
* detecting if input timestamps are non-increasing
* detecting if the order the frames come out is the same as the order
  they were inputted (meaning the implementation is reordering frames).

If the decoder reorders frames, but input buffer timestamps were not
reordered, that means the buffers has DTS and not PTS as their timestamp.
If this is the case, we use set the PTS of the outgoing frames in the
same order as they were given to the decoder.

This fixes the issue for any decoder using this base class (yay).