Shuffle things around so original file sizes are easier to access

Make original file width/height part of LightboxImage, and pass it
to the image loading function.

This adds a bit to the big steaming pile of technical debt that is
mmv.js. To be cleaned up later.

Change-Id: I84b8ae75cd1cc3e94c6cf03d174473764cfbf86f
Merge "Shuffle things around so original file sizes are easier to access"