I'm glad to see this event added, but it belongs inside the `Dispose(bool)` call.  As the docs say:

> Occurs when Dispose is called or when this object is finalized and collected by the garbage collector
http://msdn.microsoft.com/en-us/library/microsoft.xna.framework.graphics.graphicsdevice.disposing.aspx

By moving it inside the `Dispose(bool)` it is called for normal disposal and finalization.

Also it would be nice if we verify what parameters should be passed to the event and not just pass null.