Grab the corners and reshape the triangle. See http://scratch.mit.edu/discuss/topic/98866/ The barycentric test looks cheaper than this one for outlined triangles, but I think if you need filled triangles, http://scratch.mit.edu/projects/53377926/ may be marginally better.
This is a simplification of the on-the-fly-checking algorithm from http://scratch.mit.edu/projects/53377926/, the difference being that this code doesn't check for inclusion at every Y value - it only checks once with the Y value of the tested point, and it doesn't actually draw anything in this version - the testing is separated from the drawing code.
