Revision 1468

(ViewPort.calc_min_max_scales): New.
        Calculates the minimum and maximum scale values. Factored out
        of set_view_transform so that it could be used to zoom all the
        way into a single point.
(ViewPort.set_view_transform): Call calc_min_max_scales().
(ViewPort.FitSelectedToWindow): Zoom to the maximum scale
        if only a single point is selected.