Return a bounding box of the link view.
opt.useModelGeometry option is set to
true, the resulting bounding box is calculated based on the dimensions of the link model. (This means that a simplified polyline is used and portions of curved links may be
sticking out of the reported bounding box.) This behavior is similar to the
link.getBBox function – but the
linkView function transforms the bounding box to match
joint.dia.Paper translation and scaling.