h264parse: Add robustness by using AUD to detect first VCL NAL
- Fix skipsize on _update_backlog failure.
- Add robustness to AU completion detection by using AUD when present. If we've received a AUD we overwrite the first VCL NAL detection decision when the result was negative. VCL following AUD is the first VCL of next AU.
- I also change ERROR to WARNING when we receive SLICE for which we don't have SPS/PPS, to mimic previous behavior.
Fix: #3182 (closed)