Fix scaling

[gontovnik]

See #33 for more context. Removing this centering logic fixes the issue and makes the exporter rely on the AVVideoScalingModeKey.

I might be lacking some context and the proposed solution might break some scenarios that I am not aware of. I am open to any feedback ;)

[sjmueller]

Just FYI this doesn't fix some videos with transforms, especially with iOS screen recordings that have been cropped.