fix: set HEVC sync samples from IRAP VCL NALs

[gemxx]

This PR will...

Fix a HEVC fMP4 sync sample regression introduced by #7854, where parameter-set NALs could mark samples as keyframes.

A HEVC sample is now marked as sync only when its access unit contains an IRAP VCL picture, such as BLA, IDR, or CRA.

Test stream:

https://sf1-cdn-tos.huoshanstatic.com/obj/media-fe/badcase/hls-hevc-au-testset/hls-hevc-sync/index.m3u8

Why is this Pull Request needed?

Some HEVC TS streams place parameter sets before regular non-IRAP pictures:

VPS/SPS/PPS/SEI/AUD + TRAIL

This issue was introduced by #7854. After access-unit boundary handling groups those prefix NALs with the following picture, the old SPS path could still mark the sample as key:

VideoSample.key = true;

That can produce an invalid fMP4 sync sample:

wrong:   K__ [VPS, SPS, PPS, prefix SEI, AUD, TRAIL]
correct: ___ [VPS, SPS, PPS, prefix SEI, AUD, TRAIL]

TRAIL is not a random access picture. On Chrome/macOS, VideoToolbox may fail decoding when such a sample is incorrectly marked as sync:

PIPELINE_ERROR_DECODE
NSOSStatusErrorDomain Code=-12909
VTDecompressionOutputCallback

Real IRAP pictures remain sync samples:

K__ [SPS, PPS, prefix SEI, prefix SEI, AUD, IDR]

Are there any points in the code the reviewer needs to double check?

Please check the HEVC sync-sample rule:

VPS/SPS/PPS/AUD/SEI do not make a sample sync.
Only IRAP VCL pictures, such as BLA/IDR/CRA, make a sample sync.

Also note that VideoSample.key = true is still needed in IRAP branches because a sample may already have been opened by prefix NALs before the IRAP VCL NAL is parsed.

Resolves issues:

N/A

Checklist

• [ x] changes have been done against master branch, and PR does not conflict
• new unit / functional tests have been added (whenever applicable)
• API or design changes are documented in API.md

Validation performed:

• npm run build:debug
• npx prettier --check src/demux/video/hevc-video-parser.ts
• git diff --check
• Manual Safari/Chrome/Firefox macOS/Window playback verification with the affected HEVC stream

[robwalch]

cc @devoldemar