Return a window that is already displaying BUFFER.
Return nil if no usable window is found.
If ALIST has a non-nil `inhibit-same-window' entry, the selected
window is not eligible for reuse.
If ALIST contains a `reusable-frames' entry, its value determines
which frames to search for a reusable window:
nil -- the selected frame (actually the last non-minibuffer frame)
A frame -- just that frame
`visible' -- all visible frames
0 -- all frames on the current terminal
t -- all frames.
If ALIST contains no `reusable-frames' entry, search just the
selected frame if `display-buffer-reuse-frames' and
`pop-up-frames' are both nil; search all frames on the current
terminal if either of those variables is non-nil.
If ALIST has a non-nil `inhibit-switch-frame' entry, then in the
event that a window on another frame is chosen, avoid raising
that frame.