Tiled Galleries: improve display of panoramic images.

Attempt to show panoramic images either in their own rows, or such that within
a row, one other image appears on a single side while the panoramic image
occupyies the majority of the row.

Props jacklenox.



git-svn-id: https://plugins.svn.wordpress.org/jetpack/trunk@776833 b8457f37-d9ea-0310-8a92-e5e31aec5664