Fix minor errors detected by CRAN, work around Xvfb bugs

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/rgl/pkg/rgl@1197 79a1b648-8b68-254f-b20b-c0b50f5137bf
Attempt to fix linking problems in Windows

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/rgl/pkg/rgl@1198 79a1b648-8b68-254f-b20b-c0b50f5137bf