Rather than one of the input pad video info's.
The test checking this was not constraining the output frame size
to ensure that the out of frame stream was not being displayed.
We verify that all the buffers on an obscured sinkpad are skipped by overriding
the map() function in the GstVideoMeta of the buffers to set a variable when
called. We also test that the buffers do get mapped when they're not obscured.
Blame^WCredit for the GstVideoMeta map() idea goes to Tim.
https://bugzilla.gnome.org/show_bug.cgi?id=746147