base: Set prefer-passthrough on alpha only if available
This allows us to work with gst-plugins-good prior to 0.10.24 https://bugzilla.gnome.org/show_bug.cgi?id=640936
parent
2fb6f048
This allows us to work with gst-plugins-good prior to 0.10.24 https://bugzilla.gnome.org/show_bug.cgi?id=640936