Move reference images from view3d to camera
This is an initial patch, part of the proposal to use background images only for camera view: T52668
The reason to post a WIP patch is to check if the approach is generally OK, before spending time to finish it off.
- Remove 3D view background images (and panel from viewport).
- Add Camera background images (and panel to camera properties).
- Test code to call this from draw-manager.
- Add 'scale' variable (previously this wasn't possible, we can offset and rotate already, scale seems reasonable. less hassles to add now than later).
- Current patch changes view3d functions to use camera's in-place (to make the diff less noisy, the functions should be moved for the final patch).
- Draw manager uses extern to call into legacy viewport code. The function should be moved to a more general location.
- Draw manager use currently draws images on-top, even when the option to draw behind is used (needs investigation).