Dispose the ChangeQueue when closing the viewport.

Closinng can be just switching away from our display
mode integration.

If you don't dispose of the changequeue but leave it hanging
you'll make the display pipeline hang ( see https://discourse.mcneel.com/t/black-viewports-after-changequeue/84544/20?u=nathanletwory )

rhinocommon/cs/SampleCsRendererIntegration/MockingBird/MockingBirdViewport/MockingRender.cs

@@ -119,6 +119,7 @@ public void ColorPixels()
 				Thread.Sleep(10);
 				if (_shutdown) break;
 			}
+			cq.Dispose();
 		}
 
 		public event EventHandler RenderReset;