Fixes initial canvas display issues on the Mac

Now `using Winston; plot(1:10)` works as expected, with one exception.  A resize of the window, followed by a new plot gives an upside down, shifted and cropped plot, but a normal plot appears after the first resize.

I haven't been able to figure out this last problem.  `dump(c)` reports the correct sizes for the front and back surfaces right after the plot, and after resizes.

One workaround is to make Winston plot everything in a new window each time...