Robust handling of unescaped NAL units in byte stream

[kqyang]

We expect all NAL units having emulation prevention applied, i.e. sequence of 000001 should have been escaped, otherwise the NAL units cannot be extracted correctly. However, it is not always the case in field, e.g. #93.

We need more robust handling here. It is difficult and complicated to handle it perfectly. We will start with a simple algorithm and see how it goes.

Previously, whenever we see start codes 000001 we will start a new NAL unit. In the new algorithm, we will parse the NAL unit type, and only start a new NAL unit if it is a valid NAL unit type.