Return the visual class of the X display TERMINAL.
The value is one of the symbols `static-gray', `gray-scale',
`static-color', `pseudo-color', `true-color', or `direct-color'.
The optional argument TERMINAL specifies which display to ask about.
TERMINAL should 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)