Background image

[daladim]

Following #1235, here is my attempt at implementing background images.
That was relatively easy, since the rendering of images existed already.
This PR also includes an example of background image in the tour example, which has been correctly tested on Windows and Linux.

This is a basic support, background images are always stretched, and I think there should be more options in the future. I have left comments in the BackgroundImageAlignment enum to give such ideas.

I am not sure how this would affect performance, because I haven't understood yet how/if the image::Handle caches data after the first time it reads and decodes an image from the disk.