Feature/subpic
implement first part of subpicture functionality.
- the mapping between slice and subpicture works as expected, with two more configuration files added
- cfg/nonCTC-SliceConfigExamples/subpicture_4Slice2HorSubPic.cfg
- cfg/nonCTC-SliceConfigExamples/subpicture_4Slice2VerSubPic.cfg
- decoder side check works (marco: SUBPIC_DECCHECK), by masking off unwanted subpictures and write only target subpicture
- for all intra pictures, subpicture treating boundaries as picture boundary works with the decoder side check test