This fixes OpenGL errors reported for the vtkTkRenderWidget by ensuring
that Tk has fully updated before the first render.  This update problem
was missed for a long time (perhaps years) because the OpenGL errors are
only reported in debug builds, and only since the recent OpenGL changes
to VTK.

The window positioning for the vtkTkImageViewerWidget on Cocoa was also
fixed.  It was apparently never implemented.