text: Title: Convert invokes FFmpeg with redundant & conflicting arguments
  Body: **Crash reports MUST be included when reporting bugs.**
  
  **Describe the bug**
  FaceSwap convert invokes FFmpeg on the writer side with 2 sets of conflicting output codec options. The first set is generated by write_frames in imageio-ffmpeg, the second by output_params in convert's ffmpeg module. 
  
  /mnt/data/homedir/miniconda3/envs/faceswap/bin/ffmpeg -y -f rawvideo -vcodec rawvideo -s 3840x2160 -pix_fmt rgb24 -r 29.97 -i - -an **-vcodec libx264 -pix_fmt yuv420p -crf 25** -v error -vf scale=3840:2160 **-c:v libx264 -crf 23 -preset medium** /mnt/data/workspace/18/output.mp4
  
  https://github.com/deepfakes/faceswap/blob/183aee37e93708c0ae73845face5b4469319ebd3/plugins/convert/writer/ffmpeg.py#L95
  
  **To Reproduce**
  Steps to reproduce the behavior:
  1. Run a convert
  2. Inspect ffmpeg arguments with `ps aux | grep ffmpeg`
  
  **Expected behavior**
  FFmpeg invocation should not have redundant/conflicting arguments. 
  
  **Screenshots**
  If applicable, add screenshots to help explain your problem.
  
  **Desktop (please complete the following information):**
   - OS: CentOS 8
   - Python Version 3.6.8
   - Conda Version [e.g. 4.5.12]
   - Commit ID 09c7d8aca3c608d1afad941ea78e9fd9b64d9219
  