In particular, these new functions don't need to be inside the face
recognition class.  So I moved them out.  I also fixed many incorrect
copy/pasted comments and clarified parts of the example code.

landmarking models.

point face landmark models.

Made object part annotations shown on the image_display scale relative to the size of their bounding box.

faster.  The CPU path is also a lot faster as well.

CPU cores.

sub_images of anything.  Also made the resize_image() functions a little more
flexible and compatible with the other image interpolation code in dlib.