added field for pressure altitude to global position

msg/vehicle_global_position.msg

@@ -18,3 +18,4 @@ float32 epv			# Standard deviation of position vertically
 float32 terrain_alt		# Terrain altitude in m, WGS84
 bool terrain_alt_valid		# Terrain altitude estimate is valid
 bool dead_reckoning		# True if this position is estimated through dead-reckoning
+float32 pressure_alt		# Pressure altitude

src/modules/mavlink/mavlink_messages.cpp

@@ -2602,7 +2602,7 @@ protected:
 		if (updated) {
 			mavlink_altitude_t msg;
 
-			msg.altitude_monotonic = global_pos.alt;
+			msg.altitude_monotonic = global_pos.pressure_alt;
 			msg.altitude_amsl = global_pos.alt;
 			msg.altitude_local = -local_pos.z;
 			msg.altitude_relative = home.alt;