With the aim of removing `seqbasep` to remove the complicated logic for
repairing pointers within the `Strip` struct when loading files and undo
steps, this commit just moves access of the variable behind a function.
In the future the function will retrieve the list from a Strip pointer,
for now it just returns the existing pointer.
Overall motivation is that blend file pointer manipulation is incompatible
with the changes required for #127706.
Pull Request: https://projects.blender.org/blender/blender/pulls/144624
Previously code that was reading Strip data assumed that seqbasep
and channels members would stay at fixed offsets within a struct,
forever into the future. Fix this by inferring their offsets from
the file SDNA data where needed.
Actual Strip DNA layout is not changed in this commit yet.
Co-authored-by: Sergey Sharybin <sergey@blender.org>
Pull Request: https://projects.blender.org/blender/blender/pulls/142940
