Convenience function that calls Widget.queueDrawRegion on the region created from the given coordinates.
The region here is specified in widget coordinates. Widget coordinates are a bit odd; for historical reasons, they are defined as widget->window coordinates for widgets that return TRUE for Widget.getHasWindow, and are relative to widget->allocation.x, widget->allocation.y otherwise.
width or height may be 0, in this case this function does nothing. Negative values for width and height are not allowed.