Improve performance of IIIF Manifest generation

[alxp]

PR forthcoming.

The Islandora IIIF module contains a Views style plugin for generating IIIF manifests.

Many pages in Islandora repositories are stored as TIFF images using the File media type.

One requirement of IIF manifests is that each canvas (image) include the image's dimensions.

For media of Image type this is easily retrievable via a field, however TIFFs in File format are opened directly and the image file is examined on each load for each image.

We have a performance improvement internally where we add with and height fields to the File media type and store these values if they aren't already populated the first time the IIIF Manifest is generated.