Function: x-display-backing-store

Return an indication of whether X display TERMINAL does backing store.
The value may be `always', `when-mapped', or `not-useful'.
The optional argument TERMINAL specifies which display to ask about.
TERMINAL should be a terminal object, a frame or a display name (a string).
If omitted or nil, that stands for the selected frame's display.

(fn &optional TERMINAL)