The canvas (or other resource) the state points at.
Optionalregion[x, y, w, h] in canvas pixels, when the state names a part of the
canvas. Carried on the id as an xywh= fragment, which is the
Media Fragments spelling the spec uses in every region example.
A IIIF Content State 1.0 target: the canvas a deep link names, and the region within it when the link carries one.