Bug 188283 Bubbles should be z-sorted by size in Bubble charts
Biggest bubbles should be drawn first so that there is no small bubble hidden by a bigger one.
chart/org.eclipse.birt.chart.engine/src/org/eclipse/birt/chart/event/PrimitiveRenderEvent.java
Comparable
{
	/**
	 * A constant indicats a Drawing operation.
	 */
	public static final int DRAW = 1;
	/**
	 * A constant indicats a Filling operation.