video: Don't overshoot QoS earliest time by a factor of 2

By setting the earliest time to timestamp + 2 * diff there would be a difference of 1 * diff between the current clock time and the earliest time the element would let through in the future. If e.g. a frame is arriving 30s late at the sink, then not just all frames up to that point would be dropped but also 30s of frames after the current clock time.