Support for showing images in background/foreground and only in perspective/orthographic view.
Internally the depth of the image is modified in the fragment shader by setting `gl_FragDepth` explicitly.
The UI still needs some work to improve usability, see D3863 for details.
Currently there is one duplicated function, not sure how to best deduplicate it yet. (`is_image_empty_visible`)
Reviewer: fclem, brecht, campbellbarton
Differential Revision: https://developer.blender.org/D3863
