Classes implementing this interface can be used along with DefaultWindowManagerTextGUI to put some extra processing
after a window has been rendered. This is used for making window shadows but can be used for anything.
Called by DefaultWindowTextGUI immediately after a Window has been rendered, to let you do post-processing.
You will have a TextGraphics object that can draw to the whole screen, so you need to inspect the window's
position and decorated size to figure out where the bounds are
textGraphics - Graphics object you can use to draw with