De-duplicate cap faces by geometry inside Graphics_scene, so a shared wall
is stored once but referenced by every incident volume. This removes the
per-drawer dart_face map: the LCC drawer now only adds each volume's faces
between volume_begin and volume_end.
A face added without a color inherits the color of its volume; an explicit
face color still wins. Buffer_for_vao gains get_points_of_current_face and
cancel_face for the de-duplication check before a face is committed.
Rajdeep Singh
36b3f997a5