example command line to verify it:
./ffmpeg -i input.stream -vf addroi=0:0:iw/3:ih/3:-0.8 -c:v libvpx -b:v 2M tmp.webm
Signed-off-by: Guo, Yejun <yejun.guo@intel.com>
Signed-off-by: James Zern <jzern@google.com>