split-window-preferred-function

[martin rudalics]

`split-window-preferred-function' appears half-baked: In
`display-buffer' its calls are preceded by things like

          && (window_height (window) >= split_height_threshold
          ...
          && (window_height (window)
              >= (2 * window_min_size_2 (XWINDOW (window), 0))))

Set to some horizontal splitting function, splitting will be wrongly
rejected when the original window is not sufficiently high and wrongly
accepted when the window is not wide enough.  Hence it seems that we
need something like `split-width-threshold' and a way to detect how
`split-window-preferred-function' is going to split the window in order
to know which of our checks should be applied.