docstr: Return a list of CanvasBackends.
  
  Returns
  -------
  list
      A list with CanvasBackends.
code: def get_canvasses(cls):
      """Return a list of CanvasBackends.
  
      Returns
      -------
      list
          A list with CanvasBackends.
      """
      return cls._canvasses
  
imports: from __future__ import division
  from time import sleep
  import warnings
  from ..base import BaseApplicationBackend, BaseCanvasBackend, BaseTimerBackend
  from ...util import keys
  from ...util.ptime import time
  from ...gloo import gl
label_desc: Use image node_id 2 for calling the _TkInstanceManager obj's underlying member method code with example usage: obj.get_canvasses(cls) and returns: cls