The Camera Background Images uses a alpha under blending. For alpha
under blending to work correctly the framebuffer containing the result
of the render engine needs to be active
(`DefaultFramebufferList.default_fb`) and the source blend color needs
to be premultiplied with alpha.
Due to recent refactoring this wasn't the case and it seemed that the
background image was drawn in front of the scene. This patch sets the
correct state so it seems to be drawn behind the scene.
Reviewed By: fclem
Differential Revision: https://developer.blender.org/D6365
31c77326c9