[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 #12 from Sebastian Dröge (slomo) <slomo at coaxion.net> ---
I'm asking so that we can figure out how that "videocrossfade" element should
look like :)

For compositor I was thinking of adding a simple "source operator" property on
the pads, which currently would have properties for SOURCE (just copy over),
OVER (normal mode as of now), ADD (what the current crossfade property
enables).

Then we have the current default behaviour preserved, and a way to do exactly
what the crossfade property does right now but with more control over what
happens instead of hardcoding one specific way of combining things.

One thing that is not clear to me is why the current crossfade code is doing
ADD for the two pads, and then OVER for the background. Wouldn't you want the
first pad to be directly replacing the background and the other pad crossfade
into that one then? That's the reason for currently the background bleeding
through.

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