Split up rendering into two stages:  the first stage computes matrices but does no OpenGL calls, the second actually does the rendering, but doesn't affect the rgl structures.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/rgl/pkg/rgl@1156 79a1b648-8b68-254f-b20b-c0b50f5137bf