freeze_background for scenes

camera.py

@@ -41,7 +41,7 @@ class Camera(object):
         self.space_shape = (space_height, space_width)
 
     def init_background(self):
-        if self.background:
+        if self.background is not None:
             self.pixel_shape = self.background.shape[:2]
         else:
             background_rgb = color_to_int_rgb(self.background_color)

scene/scene.py

@@ -60,6 +60,11 @@ class Scene(object):
         else:
             self.camera.reset()
         self.camera.capture_mobjects(mobjects, **kwargs)
+
+    def freeze_background(self):
+        self.update_frame()
+        self.set_camera(Camera(self.get_frame()))
+        self.clear()
     ###
 
     def add(self, *mobjects):