Modify image.get_rendition() to take advantage of prefetch_related

The Picturefill polyfill makes it possible to output responsive img tags using srcset with cross browser support.

image.get_rendition() does not take advantage of prefetch_related because of the way the rendition lookup is written. (ref: models.py#L172)

Since a responsive img tag links to many renditions, rendering a lot of images greatly increases the number of database queries. I.e. to output an image gallery with 30 images using 10 possible renditions per srcset suddenly takes 300 roundtrips to the database for renditions.

It would be beneficial to use

            renditions = self.renditions.all()
            rendition = python_selection_func(renditions, filter, vary_key)

instead of

            rendition = self.renditions.get(
                filter=filter,
                focal_point_key=vary_key,
            )

so that prefetch_related could be utilized.

[zerolab]

Will mark this as closed as #8069 has been merged and is available since Wagtail 3 And Wagtail 4 also has #8584

[zerolab]

There was some work in this area - #2461 / #4856 introduce an image_url template tag (usage docs) that should help for this type of scenarios