bg-video: keep a ref of the background in the plugin
bg-video: keep a ref of the background in the plugin

As we don't want the container to own the background actor or else, with
unparented to switch betweent the video backgroud and the player, the
Mx stack and thus the VideoTexture are destroyed.

media-explorer/media-explorer,media-explorer/media-explorer,media-explorer/media-explorer,media-explorer/media-explorer