Photon: Filter image source if it seems to be a resized version of a larger image

Props @georgestephanis

See original trac ticket here:
https://plugins.trac.wordpress.org/ticket/1766

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Issue: **Photon doesn't recognize images using a filename similar to WordPress' default resized images**
Steps to reproduce:
1. Activate Jetpack and Photon on your site
2. Upload and insert an image in a new post, using the following filename: `my-image-500x500.jpg`
3. Publish your post.
4. Photon will replace the image path by `my-image.jpg?resize=500%2C500`, although `my-image.jpg` doesn't exist.


If we don't get a response when querying for the "original image", we should try to load the resized image instead.