public Rect GetInvalidRectangles( ICanvasResourceCreatorWithDpi resourceCreator )
This method is used to map rectangles from the source to output coordinate systems of an effect graph. First, InvalidateSourceRectangle(ICanvasResourceCreatorWithDpi, UInt32, Rect) is called one or more times to populate a list of what source regions have changed. Then, GetInvalidRectangles is called to map these regions into the coordinate system of the effect output.
The list of invalid regions is cleared whenever the effect is successfully drawn (i.e. a CanvasDrawingSession that it has been drawn to is closed).