FAQ: Extend part describing OpenGL/Direct3D issues

Qt Creator nowadays does a runtime check, and shows a nagging dialog
if OpenGL context cannot be created. Also, VirtualBox needs the Direct3D
support to be enabled in the Guest Addons, too.

Change-Id: Ie4cd94312a1e0061a5780eeac8124940b0bafc47
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>