[Cin] Color erase like GIMP

[Stefan de Konink]

On Tuesday, September 28, 2021 8:21:58 AM CEST, Andrew Randrianasulu via 
Cin wrote:
> well, if you have patch for ffmpeg (relatively new version) you can also
> patch Cinelerra-gg's internal copy of ffmpeg and try your filter from
> there, or add it as new blending mode as I tried (for other equations) some
> time ago (hack attached)

In my own cinGG ff_blend is not available as plugin. So... need to figure 
out that part first.

In addition; I was already warned that my code would cause issues on 420 
formats. Should check that too.

-- 
Stefan