Fix (uncatchable) traceback in GTK4 embedding example

On startup, GTK4 appears to call `draw` earlier, which causes an
uncatchable traceback:
```
Traceback (most recent call last):
  File "lib/matplotlib/backends/backend_gtk4.py", line 203, in _draw_func
    self.on_draw_event(self, ctx)
  File "lib/matplotlib/backends/backend_gtk4agg.py", line 30, in on_draw_event
    np.asarray(self.renderer.buffer_rgba()))
AttributeError: 'FigureCanvasGTK4Agg' object has no attribute 'renderer'
```
Since a real draw happens later, we just need to be sure a renderer
exists at this time.

Author: QuLogic

lib/matplotlib/backends/backend_gtk4agg.py

@@ -27,7 +27,7 @@ def on_draw_event(self, widget, ctx):
         ctx = backend_cairo._to_context(ctx)
 
         buf = cbook._unmultiplied_rgba8888_to_premultiplied_argb32(
-            np.asarray(self.renderer.buffer_rgba()))
+            np.asarray(self.get_renderer().buffer_rgba()))
         height, width, _ = buf.shape
         image = cairo.ImageSurface.create_for_data(
             buf.ravel().data, cairo.FORMAT_ARGB32, width, height)