This is part of T79273 where separating a stroke would result in
multiuser gpencil data for the original object.
Real underlying issue seems to be that gpencil multiuser objects (also
the ones created by Alt+D duplicating) have a problem evaluating
modifiers correctly [this will need further investigation].
Not sure if this is a limitation of D5470?
This patch only corrects the usercount on separating [which already fixes
the scenario reported because singleuser gpencil modifiers work
correctly].
Note: we could have also called `ED_object_add_duplicate` with the
`USER_DUP_GPENCIL` dupflag -- that would have taken care of the usercount
--, but then the whole following logic in `gpencil_stroke_separate_exec`
would need to change from **adding** layers/frames/strokes to
**removing** these.
Part of T79273
Maniphest Tasks: T79273
Differential Revision: https://developer.blender.org/D8419
4251a87bf6