gst/realmedia/rdtdepay.c: Change caps to x-rdt, because it's not x-rtp.

gst/realmedia/rdtdepay.c: Change caps to x-rdt, because it's not x-rtp.

Original commit message from CVS:
* gst/realmedia/rdtdepay.c: (gst_rdt_depay_base_init),
(gst_rdt_depay_init), (gst_rdt_depay_setcaps),
(gst_rdt_depay_chain), (gst_rdt_depay_change_state):
Change caps to x-rdt, because it's not x-rtp.
* gst/realmedia/rmdemux.c: (gst_rmdemux_add_stream),
(gst_rmdemux_parse_mdpr), (gst_rmdemux_parse_packet),
(gst_rmdemux_plugin_init):
Activate pad before adding to the element.
Return the combined stream return values after pad_alloc.