It makes it more usable to virtually move dst rect instead of scaled
src.
The issue with the latter is that it is affected by video-zoom
paramters. For example if we do `--video-pan-x=-0.5` in normal case it
will move video -50%, but if we apply video-zoom/scale the video will
float towards the middle in unintuitive way. Extreme case is when one do
--video-zoom-x=0.01, now it is impossible to move video unless you
specify huge video-pan-x value, but it is limited to [-3, 3]. So you
cannot do anything.
With this changes regardless of video scale/zoom, video-pan will keep
center of scaled video in one place.
Kacper Michajłow
7d6f9e3739