Fix image resize/resample 'bug' in Firefox

[jgehrcke]

In your demo page, when switching images with the 'menu' made by these neat little circles, the images do not just fade, but also some slight size change appears to happen which disturbs the animation.

This problem actually is a known Firefox problem:

https://bugzilla.mozilla.org/show_bug.cgi?id=745549

And there also is a solution to it:

http://support.galleria.io/discussions/problems/2549-minor-but-extremely-annoying-image-resize-artifact-after-fade-transition-on-firefox

Looks like a simple invisible shadow box helps:

box-shadow: #000 0em 0em 0em;