[Cin] ffmpeg 7.0 api changes

[Andrea paz]

In CinGG compiled with ffmpeg7 I find "strange" behavior that I do not
encounter with appimage (which has ffmpeg6).
Using a greenscreen image, as the top track and any
media as the background track, we need to apply the ChromaKey HSV
plugin. We can see that the "Alpha Offset" slider does not produce
any change. It does not work.
If we repeat the same test with appimage, the slider works normally.

Can you verify if this is also the case for you or does it depend on
me or my system?

I have also tried other plugins that use the alpha channel, and also
patchbay overlays, but everything works fine.