Add the __virtual_inheritance keyword to dummy struct when compiling with MSVC.

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.