Three new test functions covering the STAP-A (NAL type 24) packetisation
mode added to isH264IDRStart:
- LeadingIDR: STAP-A where first NAL is type 5 → true
- LeadingNonIDR: STAP-A where first NAL is SPS (type 7) → false
- Truncated: STAP-A with < 4 bytes → false, no panic