3D View: option only to show axis aligned image empties

Useful for using reference images that only make sense to see
in aligned axis-views.

This restores functionality possible with 2.7x background images.
See: T52668.

source/blender/blenkernel/intern/object.c

@@ -2985,6 +2985,15 @@ bool BKE_object_empty_image_data_is_visible_in_view3d(const Object *ob, const Re
     }
   }
 
+  if (visibility_flag & OB_EMPTY_IMAGE_HIDE_NON_AXIS_ALIGNED) {
+    float proj[3];
+    project_plane_v3_v3v3(proj, ob->obmat[2], rv3d->viewinv[2]);
+    const float proj_length_sq = len_squared_v3(proj);
+    if (proj_length_sq > 1e-5f) {
+      return false;
+    }
+  }
+
   return true;
 }