mgcp: Add a function to get media info for MGCP responses

This patch adds the get_net_downlink_format_cb() callback to provide
payload_type, subtype_name, and fmtp_extra suitable for use in a MGCP
response sent to the network. Per default, the BTS side values are
returned since these must be honoured by the net peer when sending
audio to the media gateway (unless transcoding is done).

Sponsored-by: On-Waves ehf
diff --git a/openbsc/include/openbsc/mgcp.h b/openbsc/include/openbsc/mgcp.h
index 34eb429..002dd7c 100644
--- a/openbsc/include/openbsc/mgcp.h
+++ b/openbsc/include/openbsc/mgcp.h
@@ -92,6 +92,12 @@
 typedef int (*mgcp_processing_setup)(struct mgcp_endpoint *endp,
 				     struct mgcp_rtp_end *dst_end,
 				     struct mgcp_rtp_end *src_end);
+
+typedef void (*mgcp_get_format)(struct mgcp_endpoint *endp,
+				int *payload_type,
+				const char**subtype_name,
+				const char**fmtp_extra);
+
 #define PORT_ALLOC_STATIC	0
 #define PORT_ALLOC_DYNAMIC	1
 
@@ -166,6 +172,8 @@
 	mgcp_processing rtp_processing_cb;
 	mgcp_processing_setup setup_rtp_processing_cb;
 
+	mgcp_get_format get_net_downlink_format_cb;
+
 	struct osmo_wqueue gw_fd;
 
 	struct mgcp_port_range bts_ports;