Generate Photon URLs that provide the dimensions of the intermediate sizes available on the server

When fetching images from a site, Photon gets the full-sized images, as it has no way of knowing what intermediate sizes (generated by WordPress) are available on a site. If Jetpack could generate Photon URLs that provide the dimensions of the intermediate sizes available on the server, Photon could use that information to download the appropriate one when generating the resized image. For example, Jetpack could pull the thumbnail sizes as an array and have Photon use the one that's larger than the requested size.

Solving this could improve Photon's performeance, especially on sites with many images, such as for posts with large Tiled Galleries.