JVET-R0071 Items 2-4: Inference of subpic_treated_as_pic_flag, loop_filter_across_subpic_enabled_pic_flag and single_slice_per_subpic_flag

JVET-R0071 items 2-4: cleanups on subpicture signalling (item 1 has been ported in JVET_R0156_ASPECT4)

changed title from JVET-R0071 item 2-4 to JVET-R0071 items 2-4

changed the description

changed title from JVET-R0071 items 2-4 to JVET-R0071: Items 2-4

changed title from JVET-R0071: Items 2-4 to JVET-R0071 Items 2-4: Inference of subpic_treated_as_pic_flag, loop_filter_across_subpic_enabled_pic_flag and single_slice_per_subpic_flag

merged

mentioned in commit 2e7de3b3