openh264enc: We always get a single frame as output, not a NAL

No need to accumulate NALs.
openh264enc: Properly drain the encoder on ::finish()
openh264dec: Properly drain codec on ::finish()
openh264dec: As we require NAL alignment from upstream we don't need to parse anything
openh264dec: GstVideoDecoder works on frames, not sub-frames (e.g. NALs)

And we can just pass complete AUs to the decoder without problems.
openh264dec: Don't drop videocodec frames if we can't decode them *yet*

Fixes jittery playback of streams with B frames.
openh264dec: Try to handle frame reordering

As openh264 has no way to attach any IDs to input frames that we then get on
the output frames, we have to assume that the input has valid PTS. We just
take the frame with the oldest PTS, and if there is no PTS information we take
the one with the oldest DTS.
openh264: Add FIXME comment about missing API in openh264