[Bug 797169] compositor: Remove crossfade property and implement a more extensible interface like in glvideomixer

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=797169

--- Comment #11 from Thibault Saunier <tsaunier at gnome.org> ---
(In reply to Sebastian Dröge (slomo) from comment #10)
> I think something entirely different, but something that can later be
> extended nicely with new blending operators. Something enum based :)
>
> Thibault, for your specific use case would you be fine with e.g. an API that
> kind of maps to the Cairo API? You would set (in Cairo terms) SOURCE as
> operator for the bottom pad (completely overwrites the background) with
> alpha set to "crossfade-ratio), and then the top pad would be ADD and the
> alpha would be (1-"crossfade-ratio").

Well, ideally we hide that away in a crossfade element as discussed before.
Also this way we could basically keep the current code but expose that element,
and we could have a `glcrossfade` element that has the exact same API.

> I'm not sure what behaviour you want for more than two pads (do you use
> this?), and the current implementation also seems broken as it bleeds
> through the background

The behaviour is basically "crossfade from the previous result of blending" I
currently don't use it but we thought about the API in a way that would allow
us to have 1 single compositor in the pipeline. To be honest I do not care so
much about that feature and removing it is fine to me.

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