H264Parser: Fix decoding of SPS

This also tightens up our implementation against the latest H.264 spec.

A BIG thank you to blm-ubunet@slingshot.co.nz for taking the time to read
the spec and compare it to our implementation.

Refs #11435
(cherry picked from commit efdff1ba50c1ad412b4417ffc4a1d0de07b75395)