h264: move mb_{x,y} into the per-slice context

libavcodec/dxva2_h264.c

libavcodec/h264.c

libavcodec/h264.h

libavcodec/h264_cabac.c

libavcodec/h264_cavlc.c

libavcodec/h264_direct.c

libavcodec/h264_mb.c

libavcodec/h264_mb_template.c

libavcodec/h264_mc_template.c

libavcodec/h264_mvpred.h

libavcodec/h264_slice.c

libavcodec/svq3.c

libavcodec/vaapi_h264.c