Restrict ElementImage margins to image bounds

Closes #735 (closed)

This is far from perfect and a proper rewrite of iiifUri, probably in the form of iiifUri(this.element.zone, { width: 42, height: 42 }) as well as adding a helper to restrict a bounding box to an image would be necessary to handle the situation properly.

Demo: https://restrict-margins-teklia.surge.sh/#/element/b39bcadf-e369-498a-b7e0-db9222e75e28