Change the tpad to the full duration of the transition

* When ffmpeg does an xfade, it happens during the first video (not
  half of the first and half of the second).  So the tpad has to be
  applied to the first video.

src/ffmpeg/ffmpeg.go

@@ -152,7 +152,7 @@ func MergeTempVideos(Images []string, Transitions []string, TransitionDurations
  fmt.Printf("%dth merge has transition %s and duration %f\n", i, transition, transition_duration)
  }
  //add time to the video that is sacrificied to xfade
- settb += fmt.Sprintf("[%d:v]tpad=stop_mode=clone:stop_duration=%f[v%d];", i, transition_duration/2, i)
+ settb += fmt.Sprintf("[%d:v]tpad=stop_mode=clone:stop_duration=%f[v%d];", i, transition_duration, i)
 
  //get the current video length in seconds
  video_each_length[i] = GetVideoLength(fmt.Sprintf(path.Join(tempPath, "temp%d-%d.mp4"), i, totalNumImages))