Fix inserted SEI NAL order in AVC outputs

[dsaedtler]

Description

Fixes the position where SEI NALs are inserted into AVC bitstreams.

Motivation and Context

H.264 specification section 7.4.1.2.3 "Order of NAL units and coded pictures and association to access units" clarifies that SEI NALs shall come before any VCL NALs:

• When any SEI NAL units are present, they shall precede the primary coded picture

Apparently the current method (just appending the SEIs) can cause issues with Android playback in some cases.

This is analogous to #12658 which addressed a similar issue for HEVC. AVC does not have a "suffix" SEI type, so we actually need to insert them at the correct position.

How Has This Been Tested?

Recorded some videos locally (Hybrid MP4 supports the bpm=1 option to manually enable BPM insertion for testing) and verified with a bitstream analyzer (StreamEye) that SEIs are now inserted at the correct position in the output bitstream.

My coworker who was working on diagnosing the issue on her Android phone has also confirmed that with this PR applied the issue is fixed.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have read the contributing document.
• My code has been run through clang-format.
• My code follows the project's style guidelines
• My code is not on the master branch.
• My code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.