After retrieving the affine matrix, we can apply the affine transformation to the
source image. This is done through the warpAffine function. Since we don't want to
do it in the entire image—we want to focus on our triangle—a mask can be used for
this task. This way, the last line copies only the triangle from our original image with
our just-created mask, which was made through a cvFillConvexPoly call.