Background cloneNode() fails when Background geometry BranchGroup is present.
The Geometry BranchGroup should be cloned as well, but it is only referenced.
EXCEPTION AS FOOLOWS:
javax.media.j3d.IllegalSharingException: Background: Background geometry
BranchGroup cannot be referenced by multiple Background node.
Fix is to add a call to cloneNode() to duplicateAttributes() in Background.java.