Bug 129755 [Decorators] CompositeImageDescriptor: Problem on overlay with transparent areas in PNG
bundles/org.eclipse.jface/src/org/eclipse/jface/resource/CompositeImageDescriptor.java
*/
    public ImageData getImageData() {
        Point size = getSize();
        RGB black = new RGB(0, 0, 0);
        RGB[] rgbs = new RGB[256];
        rgbs[0] = black; 
        rgbs[1] = black; 
        PaletteData dataPalette = new PaletteData(rgbs);
        imageData = new ImageData(size.x, size.y, 8, dataPalette);
        imageData.transparentPixel = 0;
        drawCompositeImage(size.x, size.y);
        for (int i = 0; i < rgbs.length; i++) {
			if (rgbs[i] == null) {
				rgbs[i] = black;
			}
		}
        return imageData;
    }
    /**
     * Return the size of this composite image.
     * <p>
     * Subclasses must implement this framework method.
     * </p>
     *
     * @return the x and y size of the image expressed as a 
     *   point object
     */
    protected abstract Point getSize();
}