Now subclassing a ghostpad with an alpha property so that
we can multiply the alpha of the frame positioning meta
and the alpha of that pad, setting it on the compositor pad.
https://bugzilla.gnome.org/show_bug.cgi?id=797169
Otherwise if frame positionner is disposed after track element has been
finalized, it will raise a critical message because we will try to
disconnect a signal handler on a freed track element object.
https://bugzilla.gnome.org/show_bug.cgi?id=766525