Significant update to backend_wx

At least on my platform, it looks as though commenting out

    > the line assert self.Ok(), "wxMemoryDC not OK to use" in
    > GraphicsContextWx.__init__() solves the problem. I am not
    > at all sure why this should be the case.

    > I'd be interested to know if this works for you too.

I commented it out (everywhere) and it ran fine. Apparently it was
hanging ...

  Two quick observations -

   - figtext renders a completely black screen
   - vertical text still broken

Otherwise the figures look fantastic!