FIX: handle the change detection differently for multicursor

There is not a singular canvas

Author: tacaswell

lib/matplotlib/widgets.py

@@ -2106,10 +2106,16 @@ def disconnect(self):
 
     def clear(self, event):
         """Clear the cursor."""
-        if self.ignore(event) or self.canvas is not self.ax.figure.canvas:
+        if self.ignore(event):
             return
         if self.useblit:
             for canvas, info in self._canvas_infos.items():
+                # someone has switched the canvas on us!  This happens if
+                # `savefig` needs to save to a format the previous backend did
+                # not support (e.g. saving a figure using an Agg based backend
+                # saved to a vector format).
+                if canvas is not canvas.figure.canvas:
+                    continue
                 info["background"] = canvas.copy_from_bbox(canvas.figure.bbox)
 
     def onmove(self, event):