Fix face extension past EOL in overlay strings

Fix face extension past EOL in overlay strings

* src/xdisp.c (face_at_pos): Pass ATTR_FILTER to
face_for_overlay_string.
* src/xfaces.c (face_for_overlay_string): Accept an additional
argument ATTR_INDEX and pass it to merge_face_ref for merging the
face at POS.  This ensures a face from buffer text will not be
merged unless it specifies the :extend attribute.  (Bug#42552)
* src/dispextern.h (face_for_overlay_string): Adjust prototype.