so they visit layers in the correct order.

Changed from ctrl to alt because otherwise it's easy to accidentally hit ctrl up and jump when you don't meant to while pruning an image list.

Made it more obvious that this file is a cmake tutorial.  Also added a few more comments about how to use cmake.

much faster when the off-diagonal vectors are all simple multiples of the
ones_matrix().

funny looking in some cases.

already cleared at the beginning

Set this because CMake search for python is wacked on ubuntu and won't look for new python otherwise.