Sets the opacity of the whole window.
This function sets 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.
The initial opacity value for newly created windows is one.
A window created with framebuffer transparency may not use whole window transparency. The results of doing this are undefined.
$windowThe window to set the opacity for.
$opacityThe desired opacity of the specified window.