FIX: do not report that webagg supports blitting

We now implement the required underlying methods (copy_from_bbox and
restore_region), but there are issues with the double buffering in nbagg,
initialization issues, and with updates to the front end being dropped.

For mpl3.4.0 over-ride the class property that checks for the required methods
until we can fix the issues with blitting.

Closes matplotlib#19701

Author: tacaswell

lib/matplotlib/backends/backend_webagg_core.py

@@ -116,6 +116,10 @@ def _timer_set_interval(self):
 
 class FigureCanvasWebAggCore(backend_agg.FigureCanvasAgg):
     _timer_cls = TimerTornado
+    # Webagg and friends having the right methods, but still
+    # having bugs in practice.  Do not advertise that it works until
+    # we can debug this.
+    supports_blit = False
 
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)