alpha: Fix sample pipeline
alpha: Fix sample pipeline

Use the zorder pad property to make sure the semitransparent
video is on top of the background.

https://bugzilla.gnome.org/show_bug.cgi?id=762809

StreamUtils/gst-plugins-good,GStreamer/gst-plugins-good,Kurento/gst-plugins-good,GrokImageCompression/gst-plugins-good,StreamUtils/gst-plugins-good,GStreamer/gst-plugins-good,GrokImageCompression/gst-plugins-good,stfl/gst-plugins-good,stfl/gst-plugins-good,GStreamer/gst-plugins-good,Kurento/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,stfl/gst-plugins-good,hizukiayaka/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,GrokImageCompression/gst-plugins-good,pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good,stfl/gst-plugins-good,StreamUtils/gst-plugins-good,hizukiayaka/gst-plugins-good,pexip/gst-plugins-good,Kurento/gst-plugins-good,Kurento/gst-plugins-good,StreamUtils/gst-plugins-good,hizukiayaka/gst-plugins-good,hizukiayaka/gst-plugins-good,Kurento/gst-plugins-good