title: Add watermark to each frame while creating video from images using ffmpeg
body: <p>I am basically creating video from opengl animation uisng <code>glreadpixels</code> and unix pipeline : 
  <code>FILE *ffmpeg = popen("/usr/local/Cellar/ffmpeg/2.5.4/bin/ffmpeg"
                       " -framerate 30"
                       " -vcodec rawvideo"
                       " -f rawvideo"
                       " -pix_fmt rgb32"
                       " -s 1080x720"
                       " -i pipe:0 -vf vflip -vcodec h264"
                       " -r 60"
                       " /Users/xamarin/Desktop/out.mp4", "w");</code>
  Is it possible to add watermark in each frame as i am creating frames for video. I know how to add watermark in existing video but i want to add watermark as i am creating video so that in on step i get video that has watermark in it. Suggest me some parameters for <code>ffmpeg</code> that can do this.</p>