Separating strokes using "selected points" didn't take into account
that points->time can be 0 and thus created points with negative
values in points->time, which should be an impossibility.
This patch fixes this in BKE_gpencil_stroke_delete_tagged_points.
Also, it makes the build modifier's new drawspeed immune
to (erroneous) negative time values, should they arise in other situations.
Reviewed By: antoniov
Differential Revision: https://developer.blender.org/D17006
9a1f0443cb