skip layers and add_prev style layers.  In particular, now in-place layers only
overwrite the gradient information in their child layer if they are operating
in in-place mode.  Otherwise, they add their gradients to their child layers.

It should also be noted that it's safe for in-place layers to overwrite
gradients when in in-place mode since their child layers are inaccessible when
in-place layers operate in in-place mode.  This prevents any other layers from
trying to add to the child layer, thereby avoiding the potability of layer
interference.  So the bug this change fixes is that, when not in in-place mode
the child layers are still accessible but in-place layers were *still*
overwriting child gradients.

using the same seed.

ago.  This fixes that and turns them back into max pooling layers.

without getting a compiler error.

Augment build-system, allow for multilib installation, support pkg-config discovery

* Define LIB_INSTALL_DIR cache variable, allowing for multilib installations
* Discover BLAS and LAPACK via pkg-config if possible
* Fix incorrect toolchain variables in "dlib/test/makefile"

what it's contract says needs to happen (and what existing code expects).

non-one based strides.

interfaces.  Also changed the default behavior when the stride isn't 1.  Now
the filters will be applied only to the "valid" part of the image.

to expose it in the final layer interface.

known at set() time.

values were used on some pixel types.

allow you to find out which device owns the memory inside these objects.