fix rendering of model previews that extend outside of a window


git-svn-id: svn://svn.tuxfamily.org/svnroot/tesseract/main@1863 b412e182-2ed4-4078-95c4-27dc59f57fa9
modelpreview shader fixes


git-svn-id: svn://svn.tuxfamily.org/svnroot/tesseract/main@1864 b412e182-2ed4-4078-95c4-27dc59f57fa9