explosion billboard optimizations


git-svn-id: svn://svn.tuxfamily.org/svnroot/tesseract/main@1900 b412e182-2ed4-4078-95c4-27dc59f57fa9
simplify explosion texgen math


git-svn-id: svn://svn.tuxfamily.org/svnroot/tesseract/main@1901 b412e182-2ed4-4078-95c4-27dc59f57fa9
yet more explosion cleanups


git-svn-id: svn://svn.tuxfamily.org/svnroot/tesseract/main@1902 b412e182-2ed4-4078-95c4-27dc59f57fa9