code: def _update_transforms(self):
          
          if len(self._fb_stack) == 0:
              fb_size = fb_rect = None
          else:
              fb, origin, fb_size = self._fb_stack[-1]
              fb_rect = origin + fb_size
              
          if len(self._vp_stack) == 0:
              viewport = None
          else:
              viewport = self._vp_stack[-1]
          
          self.transforms.configure(viewport=viewport, fbo_size=fb_size,
                                    fbo_rect=fb_rect)
docs: Update the canvas's TransformSystem to correct for the current 
          canvas size, framebuffer, and viewport.