gst-libs/gst/rtp/gstbasertppayload.*: Make timestamp offset configurable.

gst-libs/gst/rtp/gstbasertppayload.*: Make timestamp offset configurable.

Original commit message from CVS:
* gst-libs/gst/rtp/gstbasertppayload.c:
(gst_basertppayload_class_init), (gst_basertppayload_init),
(gst_basertppayload_setcaps), (gst_basertppayload_chain),
(gst_basertppayload_set_options), (gst_basertppayload_set_outcaps),
(gst_basertppayload_push), (gst_basertppayload_set_property),
(gst_basertppayload_get_property),
(gst_basertppayload_change_state):
* gst-libs/gst/rtp/gstbasertppayload.h:
Make timestamp offset configurable.