ELBG Posterizer renders differently between 21.12.3 and 22.08.1

Tested with kdenlive-21.12.3-x86_64.appimage, kdenlive-22.08.1-x86_x64.AppImage on Ubuntu 22.04.

To reproduce:

1. Open effects-stylize-21.12.3.kdenlive from erjiang/kdenlive-backcompat
2. Render out the project in both versions listed above
3. In the 22.08.1 version, the section with the ELBG Posterizer has a strange flickering effect for the black areas that the 21.12.3 version does not have.