Deformed spline should be re-evaluated after adding new feather points
Used both BKE_mask_update_display and DAG_id_tag_update because of:
- If adding feather point is happening from macro which adds point and
slides it, deformed spline should be updated immediatelly so sliding
operator will use updated feather,
- If adding happens outside of such macro, update DAG is necessary to
make needed updates in other areas (such as compositor, i.e.)
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