avoid rendering water fog over skybox when under water


git-svn-id: svn://svn.tuxfamily.org/svnroot/tesseract/main@1890 b412e182-2ed4-4078-95c4-27dc59f57fa9
render linear depth in edit mode for explicit sky texture


git-svn-id: svn://svn.tuxfamily.org/svnroot/tesseract/main@1891 b412e182-2ed4-4078-95c4-27dc59f57fa9