Add links from vignette to help pages.
Minor fixups to ?persp3d

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/rgl/pkg/rgl@1180 79a1b648-8b68-254f-b20b-c0b50f5137bf
Minor corrections to plot3d.Rd; add rgl.keepopen option to webgl chunks.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/rgl/pkg/rgl@1181 79a1b648-8b68-254f-b20b-c0b50f5137bf