reactivate scene after context lost

usage:

```
	function onDisposed()
	{
		engine.onReady = function () {
			tm.rebuildTextures(); // tm:TileManager
		}
		scene.free();  // dispose all buffers and shaders
		engine.init(); // reinit
	}
```
demo: https://dl.dropbox.com/u/1036911/h3d/contextLost/index.html

src: http://pastie.org/private/kpa615ejy6mfsrfwmj1zw