This function is supposed to be called in draw implementations for widgets that support multiple windows.
cr must be untransformed from invoking of the draw function. This function will return true if
the contents of the given
window are supposed to be drawn and false otherwise. Note that when the
drawing was not initiated by the windowing system this function will return true for all windows, so you need
to draw the bottommost window first. Also, do not use “else if” statements to check which window should be drawn.
a cairo context
the window to check.