For example:
./ffmpeg -hwaccel videotoolbox \
	-hwaccel_output_format videotoolbox_vld \
	-i foo.mp4 \
	-vf hwmap=derive_device=opencl,transpose_opencl=dir=clock,hwmap,format=nv12 \
	-c:v hevc_videotoolbox \
	-c:a copy \
	-b:v 2M -tag:v hvc1 bar.mp4
Signed-off-by: Zhao Zhili <zhilizhao@tencent.com>