Blender Git Loki

VSE: Better handling of effect strip splitting

Splitting of effect strip alone wasn't handled properly. Previously
this resulted in duplicating effect strip, and it was broken at least
from 2.79.

Change in rB8ec6b34b8eb2 was intended to allow splitting strips
individually, so it can be used as RNA API function but also so it
requires as little glue logic as possible.

This is fixed by splitting all dependent strips at once in 2 separate
ListBases for left and right strips. Strips can be finally moved into
original `ListBase`.

With this fix it is still possible to split strips individually with
little glue logic. RNA API function could return list of split strips
as well, currently at least one strip in chain will be provided so
chain can be reconstructed on python side.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D10209