Grab the corners and reshape the triangle. See http://scratch.mit.edu/discuss/topic/98866/ This works best where you are filling the triangle anyway - however there's a more efficient variation on this code for outlined or off-screen triangles at http://scratch.mit.edu/projects/53393388/
Remixed from RPGuy's project to show an alternative (possibly cheaper) way of testing if a point is inside a triangle... It fills the triangle by drawing horizontal lines, and does the inclusion test on the fly by seeing if the point is on one of those lines. The code could be further optimized, this is a quick proof of concept. (Obviously you would remove most of the redundant pen code in a real project, it was just written this way for the demo.)
