Non-conforming values of sn_subpic_id_len_minus1

[hallapur]

According to the spec, the value of sn_subpic_id_len_minus1 shall be the same for all scalable nesting SEI messages that are associated with pictures in a CVS.

VTM encoder calculates the value of sn_subpic_id_len_minus1+1 as follows:

scalableNestingSEI->m_snSubpicFlag = 1;
scalableNestingSEI->m_snNumSubpics = (uint32_t) subpictureIDs.size();
scalableNestingSEI->m_snSubpicId = subpictureIDs;
scalableNestingSEI->m_snSubpicIdLen = max(1, ceilLog2((*std::max_element(subpictureIDs.begin(), subpictureIDs.end())) + 1));

In this code the resulting value sn_subpic_id_len_minus1+1 can be different for different scalable nesting SEIs when number of subpictures is >2 and following are enabled:

SEIScalableNesting : 1
SubpicDecodedPictureHash : 1

[hallapur]

Proposed fix:
​https://vcgit.hhi.fraunhofer.de/jvet/VVCSoftware_VTM/-/merge_requests/2021