[racket] Drawing in the Canvas from outside

For the record, here's an example of that technique.

At Tue, 27 Jan 2015 11:23:29 -0700, Matthew Flatt wrote:
> You can draw on the canvas from the outside using `get-dc`. Possibly,
> though, you're seeing your drawing erased when the canvas receives a
> refresh event from the window system.
> 
> See
> 
>  http://docs.racket-lang.org/gui/canvas___.html
> 
> for more about `on-paint` and the impermanence of drawing from the
> outside.
> 
> If you need to draw from the outside and there's not a good way to
> reconstruct the drawing at any time within `paint-callback`, then one
> common strategy is: Draw to a bitmap, copy the bitmap to the canvas
> whenever the bitmap content is changed, and also have `on-paint` copy
> the bitmap to the canvas.
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