Backends may want to subclass QEglFSWindow and reimplement resetSurface()
and similar. Make it possible to do this by moving window creation to
the device integration interface, similarly to screens.

In addition to customizing the windows, some backends may want to disable
the dependency on surfaceless contexts when using offscreen windows
(i.e. pbuffer surfaces). Make this possible too.

Change-Id: Ic5a426e07f821c7a800217b8799f91770ba6a6d8
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Louai Al-Khanji <louai.al-khanji@theqtcompany.com>