Added workaround for 3delight 11.0 in IECoreRI::Renderer::worldEnd.

3delight 11.0.0 now generates a new RtContextHandle in worldEnd and whisks the other one away onto a background thread to do rerendering, so we have to talk to the new one instead. New approach tests OK in 11.0.0 and 10.0.173. Note that one test (light edits) is still failing in 11.0.0, but this so far at least appears unrelated to the context changes.
Merge pull request #131 from johnhaddon/delight11Fixes

Added workaround for 3delight 11.0 in IECoreRI::Renderer::worldEnd.