buffer: add owner private as intermediate solution

Add an owner private field where the owner of a buffer can store some extra
information. We can use this to implement most of the subclassing that happens
now. Later this will be removed and replaced by arbitrary buffer metadata.

gst/gstbuffer.h

@@ -285,6 +285,8 @@ struct _GstBuffer {
   GFreeFunc              free_func;
   GstBuffer             *parent;
 
+  gpointer               owner_priv;
+
   /*< private >*/
   gpointer _gst_reserved[GST_PADDING];
 };