Converts texture data (3 classes) into x-y coordinates. This
function is the 'heart' of most soiltexture plot functions.
TT.css2xy(tri.data, geo, css.names = NULL, text.tol = NULL,
tri.sum.tst = NULL, tri.pos.tst = NULL, set.par = FALSE,
text.sum = NULL, blr.clock = NULL)