Recalculate draggable containment before drag

When the site contents change after the draggable containment gets set
up, the container.offset() could already be outdated.
We recalculate the offsets on the mousedown event, just before actual
dragging starts.

git-svn-id: https://plugins.svn.wordpress.org/wp-photonav/trunk@1034116 b8457f37-d9ea-0310-8a92-e5e31aec5664