fix #28041: image z-order

Sets images to be as high as practical in the Z order - so they render on top of everything else, but are still selectable (elements above Measure objects in the Z order cannot be selected if they lie within the bbox for any measure).  The vtest shows two images: the one on the left (a crudely drawn "O") has a normal opaque background, while the one on the right ("T") has a transparent background.