Camera background images were not shown beneath transparent objects.
This patch performs an alpha under for background images for cycles.
In order to see the difference the Film transparency needs to be turned on.
Note that workbench and EEVEE still needs to be adapted as they don't
write the alpha value in the viewport.
This image shows the old behavior (left) vs the new behavior (right)
Side note. This implementation is already an improvement of the current behavior, what users are requesting. (Show background images underneath cycles viewport rendering.) It is clear that this patch still needs to be extended to workbench and eevee. For now that should be marked as a known limitation.