[Bug 784827] New: compositor: Add support for crossfade blending

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=784827

            Bug ID: 784827
           Summary: compositor: Add support for crossfade blending
    Classification: Platform
           Product: GStreamer
           Version: unspecified
                OS: All
            Status: NEW
          Severity: normal
          Priority: Normal
         Component: gst-plugins-bad
          Assignee: gstreamer-bugs at lists.freedesktop.org
          Reporter: tsaunier at gnome.org
        QA Contact: gstreamer-bugs at lists.freedesktop.org
     GNOME version: ---

Crossfading is a bit more complex than just having two pads with the
right keyframes as the blending is not exactly the same.

The difference is in the way we compute the alpha channel, in the case
of crossfading, we have to compute an additive operation between
the destination and the source (factored by the alpha property of both
the input pad alpha property and the crossfading ratio) basically so
that the crossfade result of 2 opaque frames is also fully opaque at any
time in the crossfading process, avoid bleeding through the layer
blending.

Some rationnal can be found in https://phabricator.freedesktop.org/T7773.

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