void render_to_drawable_alpha(GdkWindow drawable, int src_x, int src_y, int dest_x, int dest_y, int width, int height, GdkPixbufAlphaMode alpha_mode, int alpha_threshold [, GdkRgbDither dither = Gdk::RGB_DITHER_NORMAL [, int x_dither = Gdk::RGB_DITHER_NORMAL [, int y_dither = Gdk::RGB_DITHER_NORMAL]]]);