gst/rtp/: Added an H263 depayloader. Fixes #369392.

gst/rtp/: Added an H263 depayloader. Fixes #369392.

Original commit message from CVS:
* gst/rtp/Makefile.am:
* gst/rtp/gstrtp.c: (plugin_init):
* gst/rtp/gstrtph263depay.c: (gst_rtp_h263_depay_base_init),
(gst_rtp_h263_depay_class_init), (gst_rtp_h263_depay_init),
(gst_rtp_h263_depay_finalize), (gst_rtp_h263_depay_setcaps),
(gst_rtp_h263_depay_process), (gst_rtp_h263_depay_set_property),
(gst_rtp_h263_depay_get_property),
(gst_rtp_h263_depay_change_state),
(gst_rtp_h263_depay_plugin_init):
* gst/rtp/gstrtph263depay.h:
Added an H263 depayloader. Fixes #369392.
* gst/rtp/gstrtph263pdepay.c: (gst_rtp_h263p_depay_setcaps),
(gst_rtp_h263p_depay_process):
* gst/rtp/gstrtph263ppay.c: (gst_fragmentation_mode_get_type),
(gst_rtp_h263p_pay_class_init), (gst_rtp_h263p_pay_flush):
Make the H263+ pay/depayloader support H263-1998 and H263-2000
payloads.
Also alow plain H263 on the h263p payloaders. Fixes #465040.