[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 #16 from Thibault Saunier <tsaunier at gnome.org> ---
I do not remember the details of the formula tbh but what you state is not
correct, we have both alpha and crossfade-ratio so that for example you have a
clip with an alpha and a transition, it *should* bleed through because of the
alpha channel of the first clip, the formula takes that into account.

> Do you know why you need the current behaviour though, with regards to background handling? Why do you blend both pads, and then blend them with the background (instead of just overwriting the background)?

Now I remember and this behavior is expected, basically if the input clip has
alpha, that alpha needs to be kept, and in that case the background is consider
as an "input clip". We do not want the result of a transition to be opaque as
you describe.

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