Photon: Not Resizing to the new WooCommerce Thumbnail

Woocommerce changed the way product thumbnail images are handled in version 3.3, adding a new image size that is by default 100x100px. Photon doesn't pay attention to this change.

If Photon is deactivated, the product gallery thumbnails show as square:

If Photon is active, the thumbnails show as rectangles the way they used to before Woocomerce changed:

Internal ref: 1005403-zen, 1063044-zen

This is reproducible on any site. Per @jeherve, the problem appears to be linked to how some themes can declare support for `wc-product-gallery-slider`

Originally reported in 1928-gh-jpop-issues

Start by reproducing the product gallery behavior with Photon active and inactive on WooCommerce 3.3 or later, including a theme that declares support for `wc-product-gallery-slider`. Trace how Photon handles the new 100x100px WooCommerce thumbnail size; done means the gallery thumbnails retain the square dimensions when Photon is active.

Written by the indexing model from the issue text.