Line art doesn't take sequencer scene override camera into account when
computing line results, now it will try to get override camera info from
render and use that camera if line art override camera itself isn't set.
This will however not fix VSE preview line art result, since the preview
render doesn't give actual camera object, but rather uses a set of clip
planes and matrix info, thus it's probably not possible to do preview
camera override under current architecture.
YimingWu