Introduce QQuickWindowQObjectCleanupJob.

Convenience for cleaning up texture providers. Private class for now,
we can maybe make it public in 5.5.

Change-Id: I3489fde2a268bc988634a16f87ee558b9ac5dba2
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Mark QQuickFBO's node dirty after rendering the texture.

It is impossible to use in a shader effect otherwise.

Change-Id: I489e6ae43cdc3fe8f54efa6d86ead4d21831b84d
Task-number: QTBUG-40809
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>