windows: Set forward compatibility bit properly

The bit should be set when QSurfaceFormat::DeprecatedFunctions is
_not_ specified. The documentation was correct, the implementation
was not.

Change-Id: If7202d3a59d5336fff255a290b65fb4bfa7b79c9
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>
Check the thread in QOffscreenSurface

The QWindow-based fallback will not work when the QOffscreenSurface is
created outside the main thread. Show a warning in this case.

Note that it is fine to use the QOffscreenSurface instance in another
thread. It is just the creation that must happen on the main thread.

Change-Id: If80da39e610813755b9ba2f6831b187b258b8266
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>