Faster hybrid triangle fill
UPDATE; squeezed a little more out of it using a trick...
See https://scratch.mit.edu/discuss/post/908898/ :)
This new method puts more work into script execution in order to draw shorter lines that overlap much less of those parts of the triangle that have already been filled. This means it ends up just filling the remaining corners with thinner and thinner lines.
This leads to a significantly reduced render time once the triangle reaches a large enough size - hence the need for a hybrid method to keep the advantage of the original fill method for the smaller tris.
Tris drawn with the new method have black outline, so you can tell.