integrated JVET-K0206: Non-normative QPA improvements and packed YUV output
integrated JVET-K0206: Non-normative QP adaptation (QPA) improvements and packed YUV output, see also added doc/pyuv_format.pdf. ENABLE_QPA is now defined by default, to activate QPA use encoder arguments --PerceptQPA=1 --SliceChromaQPOffsetPeriodicity=1. Use Clip3() instead of max(min()) with QPs whenever possible.