codecs: h264dec: Improve the algorithm for low latency mode.

codecs: h264dec: Improve the algorithm for low latency mode.

In low_latency mode, try to bump the picture as soon as possible
without the frames disorder:
1. We can directly output the continuous non-reference frame.
2. Consider max_num_reorder_frames, which is special useful for
   I-P mode.
3. Consider the leading pictures with negative POC.
4  Output small POC pictures when non-reference frame comes.
4. Output the POC increment<=2 pictures. This is not 100% safe,
   but in practice this condition can be used.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/2373>