the asserts that DLIB_USE_CUDA must be defined.  This only matters for users who try to
install dlib and use some build system that doesn't define DLIB_USE_CUDA, and so therefore need
dlib/config.h to define it for them.

With C++17 enabled on VS15.6+, this triggered a nodiscard warning which led to failed builds when warnings as errors is enabled.

have_same_dimensions() to be uncallable for images, so that's fixed now too.

particular, it wasn't being used in some parts of the Python API.  So this
change makes the HOG detector run like 10% faster for instance.

specific to images is only called on images.  So this fixes a bug where the
image version of have_same_dimensions() was being called for tensors and
therefore only looking at the nr and nc dimension while ignoring the others.
This caused some parts of the DNN tooling to malfunction.

limit on pixel chain recursion depth.