rgl.bg now returns id of background (as advertised)

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/rgl@1147 79a1b648-8b68-254f-b20b-c0b50f5137bf
Make it invisible

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/rgl@1148 79a1b648-8b68-254f-b20b-c0b50f5137bf