I know this post question is old, but answering for helping others,
Vec2F scale = new Vec2F(targetScale, targetScale * targetImg.height() / targetImg.width());
current_video_renderer.render(projectionMatrix, targetInstance.pose(), scale);
You can modify scale value. for e.g.
Vec2F scale = new Vec2F(targetScale, targetScale * video.player.size().data / video.player.size().data);
Or any other way you want. Just remember that the first parameter is for the width of video and the second is for height (obviously).