There was a case where we wouldn't convert edit curves correctly,
specifically when the user created an edit curve
(using "Curve Editing") and then changes the stroke outside of
curve editing. This invalidates the edit curve, but does not
deallocate it.
The conversion assumed that any allocated edit curve is valid.
Now we check for the `GP_CURVE_NEEDS_STROKE_UPDATE` flag
to skip invalid edit curves (and use the stroke instead).
8c9df38eb9