Bug 202316 [Forms] Add gradient sharing support for FormHeading
bundles/org.eclipse.ui.forms/src/org/eclipse/ui/internal/forms/widgets/FormImages.java
int realtheight, int theight, int marginHeight) {
		checkHashMaps();
		ImageIdentifier id = new ImageIdentifier(display, color1, color2, realtheight, theight, marginHeight);
		ImageReference result = (ImageReference) images.get(id);
		if (result != null && !result.getImage().isDisposed()) {
			result.incCount();
			return result.getImage();
		}
		Image image = createGradient(display, color1, color2, realtheight, theight, marginHeight);
		images.put(id, new ImageReference(image));
		ids.put(image, id);
		return image;
	}
	
	public boolean markFinished(Image image) {
		checkHashMaps();
		ImageIdentifier id = (ImageIdentifier)ids.get(image);
		if (id != null) {
			ImageReference ref = (ImageReference) images.get(id);
			if (ref != null) {
				if (ref.decCount()) {
					images.remove(id);
					ids.remove(ref.getImage());
					ref.getImage().dispose();
					validateHashMaps();
				}
				return true;
			}
		}
		return false;