- create a "dummy" VPS with ID 0 on first access in ParameterSetManager
- actually use the "dummy" VPS
- improve DecLib::m_vps handling (may still fail if multiple VPSs are in the bitstream)

(1) move frame_only_constraint_flag and single_layer_constraint_flag outside the GCI syntax structure into the PTL syntax structure
(2) rename single_layer_constraint_flag to multi_layer_enabled_flag
(3) change ptl_alignment_zero_bit to ptl_reserved_zero_bit