[qt-iOS] QOffsreenSurface::isValid() always return false

Start from qt 5.3.0 under iOS, QOffscreenSurface::isValid() will always return false even if it has been created.
I think the reason for this is:
From qt 5.3.0, QIOSIntegration::createPlatformOffscreenSurface() will create a QPlatformOffscreenSurface object as the platform offsreen surface, and QPlatformOffscreenSurface::isValid() always return false.