Resize root window WINDOW vertically by DELTA lines.
If DELTA is less than zero and we can't shrink WINDOW by DELTA
lines, shrink it as much as possible. If DELTA is greater than
zero, this function can resize fixed-size windows in order to
recover the necessary lines.
Return the number of lines that were recovered.
This function is only called by the minibuffer window resizing
routines. It resizes windows proportionally and never deletes
any windows.