will allow custom FFmpeg flags to be set on source videos (including base
video) and on output video. For example, scaling will be possible on all
source videos and images, and on the final output video. Much more will be
possible as well. This change simply introduces the '[N:v]null[videoN]' chain
terminology to overlay images (where 'N' is a number starting from zero) to
overlay images. This was already present in overlay video and on the base
video. The 'null' inside of this term will then be possibly modified with
scale properties etc.
