add loading of texture arrays from a list of files and fix layer concat bug

NOTE: In Image::uploadTo I added the layers to byte count computation. So right now all layers get uploaded at the same time with one vkBufferImageCopy instance. It works fine on the GTX 1080Ti, GTX 770 and MX150. But in this example multiple instances are used (one per layer). So I'm not sure if my solution only works on Nvidia cards.