compositor: Source operation with AYUV wrongly computes output alpha channel

Doing:

gst-launch-1.0 videotestsrc pattern=blue ! compositor name=c sink_0::operator=source ! video/x-raw,format=AYUV ! videoconvert ! glimagesink

leads to pixels with alpha_channel=0xa0 not 0xff as expected, you can simply see it doing

gst-launch-1.0 videotestsrc pattern=blue ! compositor name=c sink_0::operator=source ! video/x-raw,format=AYUV ! videoconvert ! compositor ! glimagesink

Where the checkboard show up where it shouldn't

The orc function is the same as the ARGB variants, and interestingly this works just fine:

gst-launch-1.0 videotestsrc pattern=blue ! compositor name=c sink_0::alpha=1.0 sink_0::operator=source ! video/x-raw,format=ARGB ! videoconvert ! glimagesink

See https://gitlab.gnome.org/GNOME/pitivi/issues/2321