add box-shadow to over-sized content

[mixmix]

The intention here is to suggest oversized content is subducting.

[mixmix]

there's a known problem with this - when there's an image in the box and the image is white, what I've made looks shit (i.e. the image breaks the illusion and doesn't look like it's sliding under at all:

Other images look fine:

This explains what I think the problem is. I tried implementing, but had trouble :
http://stackoverflow.com/questions/21414925/why-doesnt-inset-box-shadow-work-over-images
Open to thoughts

[mixmix]

moving this to : patchkit/patchkit-msg-view#1