initial blend_func_extended usage


git-svn-id: svn://svn.tuxfamily.org/svnroot/tesseract/main@1549 b412e182-2ed4-4078-95c4-27dc59f57fa9
fix detection of ARB_blend_func_extended extension


git-svn-id: svn://svn.tuxfamily.org/svnroot/tesseract/main@1550 b412e182-2ed4-4078-95c4-27dc59f57fa9