Always grow the stack in push_new_viewport()
Always grow the stack in push_new_viewport()

We didn't grow the stack if the new vbox would be the same as the last
one (i.e. when the passed vbox == None).  Now we always grow the
stack.  The idea is that eventually we'll be able to automatically
restore the last viewport's transform on Drop.

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg