Shared rendering settings

--exclude

This is unlikely to work for all clients, but as a discussion piece, this PR has a test showing (a part of) the behavior we'd like to see for http://trac.openmicroscopy.org.uk/ome/ticket/12145. Brief testing in web showed a second rendering def be generated *regardless* so we'll have to dive in to the clients and see precisely which methods are being called and how best to make them create rendering settings lazily.

/cc @jburel @will-moore @chris-allan