Add support for pps_single_slice_per_subpic_flag in subpicMergeApp

This adds support for pps_single_slice_per_subpic_flag in subpicMergeApp so that the entire slice layout is not coded if not needed. As a side effect more subpicture layouts are enabled to be done easily. One such layout is one subpic on the left and 2 or more small subpics on the right in a column.