def do_resize(self):
pass<EOL>
React to a change in the widget's size.

Only widgets that actually draw things need to implement this method.  
If the widget has children widgets that it may need to redistribute 
space between, it should do that in `do_resize_children()`.

However, keep in mind that this method is called before the widget is 
drawn and may be called after it's been undrawn, so any vertex lists 
created in the draw function may or may not exist yet/anymore.  If 
those vertex lists don't exist yet, there's nothing this function needs 
to do.  The `_draw()` function will be called when the widget's ready 
to draw, and at that point the vertex lists should be created with the 
right size and shape.