Initialize m_numSubPic for unavailable pictures.

See merge request !2784

A pointer to a current SPS is already stored within Picture.
No need to add a new member `m_numSubPic`

Set m_numSubPic to a value signaled in SPS for the layer.
Otherwise an error is triggered when an unavailable picture
happens to be in the reference list for GDR picture partitioned into
more than 1 sub-pictures.

Error message is: "TMVP cannot be enabled in pictures
that have no reference pictures with the same resolution"

Define default value for Picture::m_numSubPic

See merge request !2781

Fixes issues with certain GDR conformance bitstreams

Fix #1648 on sps_chroma_horizontal/vertical_collocated_flag usage in case of multilayer

See merge request jvet/VVCSoftware_VTM!2771

# Conflicts:
#   source/Lib/CommonLib/InterPrediction.cpp

Fix a regression in RPL parsing in Multilayer case

See merge request !2749

Fix #1649 on rprConstraintsActiveFlag

See merge request jvet/VVCSoftware_VTM!2772

Fix #1645 on sbtmvp enabling condition in CU-level in case of multilayer profile.

See merge request jvet/VVCSoftware_VTM!2768

Fix #1646 on the condition of using tempMV for SbTMVP

See merge request !2769

JVET AI0061 Proposal 1: signal a NNPFC prompt for spatial extrapolation

See merge request !2773

JVET-AG0322: Modality Information SEI

See merge request !2765

# Conflicts:
#   source/Lib/CommonLib/TypeDef.h

Fix #1647 on adding HMVP candidates to AMVP list (multilayer with >2 layers)

See merge request !2770

Clean up unregistered user data SEI

See merge request !2777

Fix #1643 on different pps.conformanceWindows when using --UpscaledOutput=1 with RPR

See merge request !2767

JVET-AI0061-PROPOSAL2: Ensure spatial extrapolation works correctly (Option 1)

See merge request !2774

JVET-AI0059: Define persistence scope for text description information SEI to be purpose specific

See merge request !2766

Workaround for VLCReader::xReadString for Visual C++

See merge request !2775

The current code misses the first character of the string for some reason. Maybe a compiler problem.
Using a local variable fixed this.