If t, resize window combinations proportionally.
If this variable is nil, splitting a window gets the entire screen space
for displaying the new window from the window to split. Deleting and
resizing a window preferably resizes one adjacent window only.
If this variable is t, splitting a window tries to get the space
proportionally from all windows in the same combination. This also
allows to split a window that is otherwise too small or of fixed size.
Resizing and deleting a window proportionally resize all windows in the
same combination.
Other values are reserved for future use.
This variable takes no effect if `window-combination-limit' is non-nil.