Fixed GtkViewport to set adjustments properly for width-for-height widgets

Fixed GtkViewport to set adjustments properly for width-for-height widgets

Make GtkViewport calculate widget-for-height as well as height-for-width
cases when setting the scroll adjustment values, also base the scrolling
on the minimum size instead of the natural size in the interest of showing
as much content as possible when the viewport is smaller than the natural
size.