Avoid potential infinite loop if button is released by ActionListener

The readAndDispatch() method that is implicitly called in the
PaletteScrollBar might send a "Mouse Released" event while processing
the "Mouse Pressed" event.

This then causes the timer to never be cancelled, leading to an infinite
scrolling.

Closes #733

Author: ptziegler

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

@@ -58,7 +58,13 @@ public Graphics getGraphics(Rectangle r) {
 
 		// canvas.update() paints too much and only works on Windows. Use
 		// readAndDispatch() to only paint the redraw() event.
-		canvas.getDisplay().readAndDispatch();
+
+		// Running readAndDispatch() might fire a mouseReleased() event while processing
+		// a mousePressed() in e.g. the palette scrollbar. See:
+		// https://github.com/eclipse-gef/gef-classic/issues/733
+		if (Animation.isAnimating()) {
+			canvas.getDisplay().readAndDispatch();
+		}
 		return null;
 	}