video/cdgdec: Box CdgInterpreter state to prevent GObject private size to become too big

Only 64k are allowed for the sum of all private instance structs in the class hierarchy, as well as for the public instance structs.

The CdgInterpreter itself is huge and adding just another two integers to GstVideoDecoderPrivate in libgstvideo is causing the limit to be reached, so let's allocate it in a separate memory area.