ENH: Handle redraw edge case.

You can call redraw before the first draw event so that the background never gets saved. In this case, skip blitting so we don't need the background image.

skimage/viewer/canvastools/base.py

@@ -23,6 +23,7 @@ def __init__(self, ax, on_move=None, on_enter=None, on_release=None,
                  useblit=True):
         self.ax = ax
         self.canvas = ax.figure.canvas
+        self.img_background = None
         self.cids = []
         self._artists = []
         self.active = True
@@ -76,7 +77,7 @@ def redraw(self):
 
         This method should be called by subclasses when artists are updated.
         """
-        if self.useblit:
+        if self.useblit and self.img_background is not None:
             self.canvas.restore_region(self.img_background)
             self._draw_artists()
             self.canvas.blit(self.ax.bbox)