FIX: set internal flags first in FigureCanvasBase

If interactive mode is on using the model where every time any artist is
invalidated/marked as stale a `draw_idle` is triggered and the user is
using a non-Agg based backend, saving a png will result in a draw_idle
call triggered from inside the __init__ method of `FigureCanvasBase`
which then fails because the full object has not been set up (this is
not a problem using the IPython hooks because the stale state is only
checked once when all user code has completed executing).

closes matplotlib#5094

lib/matplotlib/backend_bases.py

@@ -1678,6 +1678,14 @@ class FigureCanvasBase(object):
                          'Tagged Image File Format')
 
     def __init__(self, figure):
+        # put this first to make sure that we prevent any attempts to
+        # re-draw while we are constructing the canvas.
+        # See issue #5094, this only trigged on saving a png when:
+        #  - using a non-agg backend
+        #  - in interactive mode
+        #  - not bunching `draw_idle` calls (ex not in IPython)
+        self._is_idle_drawing = True
+
         figure.set_canvas(self)
         self.figure = figure
         # a dictionary from event name to a dictionary that maps cid->func