Improve RIIIF performance

[nhomenda]

Descriptive summary

Locate and improve known areas of poor RIIIF performance.

Rationale (for feature request only)

Default IIIF viewer is pretty slow compared with other options.

Related work

Discussed in Serving IIIF Requests in Samvera

[mark-dce]

There is a new release of RIIIF to RubyGems that includes VIPS support: https://rubygems.org/gems/riiif/versions/2.8.0

This unblocks #7289 once it is refactored to point to the new release.