Modify one constraint related to leading picture that was adopted for VVC. Other constraints for leading pictures have been implemented during HEVC time. "If field_seq_flag is equal to 0 and the current picture is a leading picture associated with an IRAP picture, it shall precede, in decoding order, all non-leading pictures that are associated with the same IRAP picture. Otherwise, let picA and picB be the first and the last leading pictures, in decoding order, associated with an IRAP picture, respectively, there shall be at most one non-leading picture preceding picA in decoding order, and there shall be no non-leading picture between picA and picB in decoding order."
requested to merge hendry197/VVCSoftware_VTM:JVET-O0147_LeadingPictureCheckings into master