Multi-View: Fix - stereo 3d background image in viewport not working

The functionality was there, but the UI was missing, since I introduced
the use_mutliview option in the Image datablocks.

Note: when opening the image via the background image UI it has its
views data mangled. I'll look at that. Meanwhile the fix is to toggle
Use Multi-View manually in the UI.

release/scripts/startup/bl_ui/space_view3d.py

@@ -3236,6 +3236,7 @@ class VIEW3D_PT_background_image(Panel):
         layout = self.layout
 
         view = context.space_data
+        use_multiview = context.scene.render.use_multiview
 
         col = layout.column()
         col.operator("view3d.background_image_add", text="Add Image")
@@ -3273,6 +3274,19 @@ class VIEW3D_PT_background_image(Panel):
                         box.template_image(bg, "image", bg.image_user, compact=True)
                         has_bg = True
 
+                        if use_multiview and bg.view_axis in {'CAMERA','ALL'}:
+                            box.prop(bg.image, "use_multiview")
+
+                            column = box.column()
+                            column.active = bg.image.use_multiview
+
+                            column.label(text="Views Format:")
+                            column.row().prop(bg.image, "views_format", expand=True)
+
+                            sub = column.box()
+                            sub.active = bg.image.views_format == 'STEREO_3D'
+                            sub.template_image_stereo_3d(bg.image.stereo_3d_format)
+
                 elif bg.source == 'MOVIE_CLIP':
                     box.prop(bg, "use_camera_clip")