Add Null check before assigning coordinateMapper

This commit adds a null check before assigning the coordinate mapper in
the Device::init since it might already be assigned in coordinateMapper.

Fixes #1686
contributes to #62 and #127

bundles/org.eclipse.swt/Eclipse SWT/win32/org/eclipse/swt/widgets/Display.java

@@ -2772,7 +2772,9 @@ protected void init () {
 	// Field initialization happens after super constructor
 	controlByHandle = new HashMap<>();
 	this.synchronizer = new Synchronizer (this);
-	this.coordinateSystemMapper = new SingleZoomCoordinateSystemMapper();
+	if (this.coordinateSystemMapper == null) {
+		this.coordinateSystemMapper = new SingleZoomCoordinateSystemMapper();
+	}
 	super.init ();
 	DPIUtil.setDeviceZoom (getDeviceZoom ());