Support reading quads

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/rgl/pkg/rgl@1190 79a1b648-8b68-254f-b20b-c0b50f5137bf
Handle arbitrary polygons in readOBJ

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/rgl/pkg/rgl@1191 79a1b648-8b68-254f-b20b-c0b50f5137bf