gst/realmedia/rdtdepay.*: Use new RDT parsing helper functions.

gst/realmedia/rdtdepay.*: Use new RDT parsing helper functions.

Original commit message from CVS:
* gst/realmedia/rdtdepay.c: (gst_rdt_depay_class_init),
(gst_rdt_depay_finalize), (gst_rdt_depay_setcaps),
(gst_rdt_depay_push), (gst_rdt_depay_handle_data),
(gst_rdt_depay_chain), (gst_rdt_depay_change_state):
* gst/realmedia/rdtdepay.h:
Use new RDT parsing helper functions.
Copy discont flags correctly.
Push the header from the chain function instead of the setcaps function.
Copy incomming timestamp to the output buffers instead of doing magic
with the RDT timestamps.