Force repaint in NativeGraphicsSource to fix broken animation

When the FigureCanvas is created with SWT.DOUBLE_BUFFERED, animations
are not painted correctly. This is because an instance of
NativeGraphicsSource is used internally, which doesn't paint
synchronously.

Because the animation is done inside the UI thread, this paint operation
is only processed after the animation is done.

Resolves eclipse-gef#376

org.eclipse.draw2d/src/org/eclipse/draw2d/NativeGraphicsSource.java

@@ -1,5 +1,5 @@
 /*******************************************************************************
- * Copyright (c) 2005, 2010 IBM Corporation and others.
+ * Copyright (c) 2005, 2024 IBM Corporation and others.
  *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Public License 2.0 which is available at
@@ -56,6 +56,9 @@ public Graphics getGraphics(Rectangle r) {
 
 		// canvas.update();
 
+		// canvas.update() paints too much and only works on Windows. Use
+		// readAndDispatch() to only paint the redraw() event.
+		canvas.getDisplay().readAndDispatch();
 		return null;
 	}