Don't access private renderer attributes in tkagg blit.

`buffer_rgba()` basically does the same from the point of view of
`_backend_tk.blit` (which will extract the buffer object from
`renderer._renderer` otherwise).

Restricting oneself to public renderer API makes it easier to use
mplcairo to patch out the agg renderer.

Author: anntzer

lib/matplotlib/backends/backend_tkagg.py

@@ -10,8 +10,8 @@ def draw(self):
         self.blit()
 
     def blit(self, bbox=None):
-        _backend_tk.blit(
-            self._tkphoto, self.renderer._renderer, (0, 1, 2, 3), bbox=bbox)
+        _backend_tk.blit(self._tkphoto, self.renderer.buffer_rgba(),
+                         (0, 1, 2, 3), bbox=bbox)
 
 
 @_BackendTk.export