Encoder improvement: use IDR_N_LP NAL unit type if GOP does not specify reordering of pictures

Karsten Suehringrequested to merge
ksuehring/VVCSoftware_VTM:idr_nlp into master

In case there is no reordering in the GOP, leading pictures cannot occur withing the VTM encoding process (e.g. LDB/P configurations).

Edited by Frank Bossen

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