Off-screen rendering support - popup windows and native dropdown menus not yet working

```
In CEF 3 Windows and Mac already support off-screen rendering,
soon Linux will also be supported, see comment #126:

https://code.google.com/p/chromiumembedded/issues/detail?id=518#c126
```

Original issue reported on code.google.com by `czarek.t...@gmail.com` on 27 Jun 2013 at 5:46
