Windows: Effect Vignette and Saturation lead to wrong rendered output.

STEPS TO REPRODUCE

1. add 2 clips to the timeline
2. clip no.1: add vignette effect
   
3. clip no.2: add saturation effect
   
4. Render the timeline

OBSERVED RESULT

• Timeline playback is OK
• Vignette effect result in a small hole (https://www.reddit.com/r/kdenlive/comments/id4val/vignette_wtf_is_going_on_am_i_missing_something/)
  
• Saturation effect result in black&white clip (https://kdenlive.org/en/2020/08/kdenlive-20-08-is-out/#comment-5029)
  

ADDITIONAL INFORMATION

The effect rendering works properly under 20.04.1
Workaround: set system language to a language which has dot as decimal separator.

see here: https://bugs.kde.org/show_bug.cgi?id=425978