lavu: add FF_CEIL_RSHIFT and use it in various places.

libavcodec/ffv1dec.c

libavcodec/ffv1enc.c

libavcodec/mimic.c

libavcodec/mjpegdec.c

libavcodec/snow.c

libavcodec/utils.c

libavfilter/deshake_opencl.c

libavfilter/vf_gradfun.c

libavfilter/vf_tinterlace.c

libavutil/common.h

libavutil/frame.c

libavutil/imgutils.c

libswscale/rgb2rgb_template.c

libswscale/swscale.c

libswscale/swscale_unscaled.c

libswscale/utils.c

libswscale/x86/rgb2rgb_template.c