add subsceneList, improve examples, make next3d() somewhat robust against changes.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/rgl@1064 79a1b648-8b68-254f-b20b-c0b50f5137bf
Fix clear3d(type="sub") bug -- currentSubscene wasn't being updated

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/rgl@1065 79a1b648-8b68-254f-b20b-c0b50f5137bf