JVET-S0050: GCI related cleanups
- Signal new GCI flags no_virtual_boundaries_constraint_flag and no_explicit_scaling_list_constraint_flag
- Constrain the value of one_subpic_per_pic_constraint_flag, one_slice_per_pic_constraint_flag and no_aps_constraint_flag
- Remove all constraints that require GCI fields to be equal to a value that imposes a constraint