Export function for drawing to GTK DrawingArea.

[masterzim]

Why didnt allow function for direct drawing on canvas? There is
updateCanvas function in Graphics.Rendering.Chart.Gtk module but it
doesn't export this function. Until I looked at the source, I did not
even know about the existence of such a function. Therefore, for such
purposes, I renders the chart in the file and then load it, but in
this case it was very inconvenient to implement scaling when resizing
the window. I think this feature must be opened for public use.

PS. Thank you for this wonderful library.

[Tim Docker]

The only reason this function wasn't exported is that I thought
that it was short and hence easy to reproduce/modify in client
code.

But there's no reason not to export it, so I've just pushed
a patch to do so.

Tim