Gets private data set on the picture via gst_vp8_picture_set_user_data() previously.
Sets user_data on the picture and the #GDestroyNotify that will be called when
the picture is freed.
If a user_data was previously set, then the previous set notify will be called
before the user_data is replaced.
a #GDestroyNotify
Create new #GstVp8Picture
Create new #GstVp8Picture