VTM doesn't start decoding on GDR with ph_recovery_poc_cnt = 0

[jack.enhorn]

VTM decoder doesn't check for GDR pictures with ph_recovery_poc_cnt = 0 as valid starting pictures.

Possible solution is to extend check for CRA to include GDR in DecLib::isRandomAccessSkipPicture:

...
else if (m_pocRandomAccess == MAX_INT) // start of random access point, m_pocRandomAccess has not been set yet.
  {
    if (m_apcSlicePilot->getNalUnitType() == NAL_UNIT_CODED_SLICE_CRA || ( m_apcSlicePilot->getNalUnitType() == NAL_UNIT_CODED_SLICE_GDR && m_apcSlicePilot->getPicHeader()->getRecoveryPocCnt() == 0 ) )
    {
      // set the POC random access since we need to skip the reordered pictures in the case of CRA/CRANT/BLA/BLANT.
      m_pocRandomAccess = m_apcSlicePilot->getPOC();
    }
...

[fbossen]

Fixed in ​https://vcgit.hhi.fraunhofer.de/jvet/VVCSoftware_VTM/-/merge_requests/1960