Returns the opacity of the whole window.
This function returns the opacity of the window, including any decorations.
The opacity (or alpha) value is a positive finite number between zero and one, where zero is fully transparent and one is fully opaque. If the system does not support whole window transparency, this function always returns one.
The initial opacity value for newly created windows is one.
$windowThe window to query.
floatThe opacity value of the specified window.