Implement HighDPI scaling for Zest figures

This continues eclipse-gef#770 and
implements HighDPI scaling for Zest graphs. If the corresponding
"draw2d.disableAutoscale" system property is set, a ZoomChange listener
is added to the Graph that automatically scales the root figure to match
the display zoom.

Author: ptziegler

org.eclipse.zest.core/src/org/eclipse/zest/core/widgets/Graph.java

@@ -74,6 +74,7 @@
 import org.eclipse.draw2d.geometry.Point;
 import org.eclipse.draw2d.geometry.Rectangle;
 import org.eclipse.draw2d.internal.FileImageDataProvider;
+import org.eclipse.draw2d.internal.InternalDraw2dUtils;
 
 /**
  * Holds the nodes and connections for the graph.
@@ -239,6 +240,10 @@ public void dispatchMouseMoved(org.eclipse.swt.events.MouseEvent me) {
 		this.zoomListener = new ZoomGestureListener();
 		this.rotateListener = new RotateGestureListener();
 
+		if (InternalDraw2dUtils.disableAutoscale) {
+			this.addListener(SWT.ZoomChanged, event -> this.rootlayer.setScale(event.detail));
+		}
+
 		this.addPaintListener(event -> {
 			if (shouldSheduleLayout) {
 				applyLayoutInternal(true);