Tiled Gallery: Pass Carousel URLs through Photon.

Use `wp_get_attachment_image_src` instead of `wp_get_attachment_url` for `data-orig-file` in Tiled Galleries to trigger `filter_image_downsize` in Photon. This way the full image link in Carousel will point to an image served by Photon.

Addresses https://github.com/Automattic/jetpack/issues/522.