New QWindow-type Class for Threaded Rendering

Hi QT team,

It seems that a QWindow is treated as a widget when it comes to multi-threaded applications while QOffscreenSurface is treated as a QObject even though they should both have the ability to be transferred to and live on a non-GUI thread.

The issue with having a QWindow on the GUI thread is that no matter how efficiently you render resources on a helper QOffscreenSurface, a QWindow's performance is subject to the GUI workload. 

For example in an application with 100+ widgets that a user is constantly interacting with, this will result in unstable performance (e.g. fps). 

My suggestion is to have a class similar to QWindow that can be transferred to a non-GUI thread. 

Cheers!