Better Detection for Thumbnail Sizes

[markpbaggett]

When a thumbnail is not on the canvas, Clover attempts to use the the full size image of the canvas. This can be less than ideal when the canvases are very large / not pre-cached and even worse when the canvas is so big that the underlying image struggles to generate the full size image. 🤦🏻

For instance, here is a multi-canvased work where the underlying original images are 400 ppi and about 50 inches long:

https://api.library.tamu.edu/iiif-service/dspace/presentation/1969.1/128911

Instead of grabbing the full image, it'd be nice for a preferred image size to be pulled.