Handle "Zoomed image" download links in book mode

[jkeck]

We currently do not render the "Zoomed image" link in book mode. The current method for getting the current viewer bounds (OSDViewer.viewer.viewport.getBounds()) doesn't work as-is in Book mode (e.g. it ends up giving us a wildly incorrect x parameter).

More info available in sul-dlss/sul-embed#983 (comment)

We have a few options:

1. "Stitch" together any visible images between two canvases into one image (preferred by @ggeisler)
2. Render a link in the download under each canvas section that would download the visible section for that particular canvas.
   • If we do this, then we need to know what to do when one canvas is not visible (e.g. not display a link)
3. Other possibilities...?

[ggeisler]

For option #2:

If we do this, then we need to know what to do when one canvas is not visible (e.g. not display a link)

Yes, I would just provide a download link for the image that is visible and omit it for the other one.