vtdec: Fix DPB size calculations not taking values from SPS into account

Fixes #1503 (closed)

Currently vtdec uses simplified calculations for its reorder queue (DPB) length. While they're good enough most of the time, in more complicated cases they can be completely wrong, which is exactly what happens in #1503 (closed) - current code calculates 1, while that file (as read from the SPS) needs 4 to ensure enough space to store the 'future' frames and reorder before sending them out.

This MR attempts to use the codecparsers lib to parse SPS and retrieve the values we need in this case, mimicking what's already done in the GstH264Decoder class used by stateless decoders.