Add clipping.  

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/rgl@1176 79a1b648-8b68-254f-b20b-c0b50f5137bf
Fix bboxdeco code; update vignette

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/rgl@1177 79a1b648-8b68-254f-b20b-c0b50f5137bf