handle an incoming mavlink packet

def mavlink_packet(self, m):
        '''handle an incoming mavlink packet'''

        if not self.mpstate.map:
            # don't draw if no map
            return

        if m.get_type() != 'GIMBAL_REPORT':
            return

        needed = ['ATTITUDE', 'GLOBAL_POSITION_INT']
        for n in needed:
            if not n in self.master.messages:
                return

        # clear the camera icon
        self.mpstate.map.add_object(mp_slipmap.SlipClearLayer('GimbalView'))

        gpi = self.master.messages['GLOBAL_POSITION_INT']
        att = self.master.messages['ATTITUDE']
        vehicle_dcm = Matrix3()
        vehicle_dcm.from_euler(att.roll, att.pitch, att.yaw)

        rotmat_copter_gimbal = Matrix3()
        rotmat_copter_gimbal.from_euler312(m.joint_roll, m.joint_el, m.joint_az)
        gimbal_dcm = vehicle_dcm * rotmat_copter_gimbal

        lat = gpi.lat * 1.0e-7
        lon = gpi.lon * 1.0e-7
        alt = gpi.relative_alt * 1.0e-3

        # ground plane
        ground_plane = Plane()

        # the position of the camera in the air, remembering its a right
        # hand coordinate system, so +ve z is down
        camera_point = Vector3(0, 0, -alt)

        # get view point of camera when not rotated
        view_point = Vector3(1, 0, 0)

        # rotate view_point to form current view vector
        rot_point = gimbal_dcm * view_point

        # a line from the camera to the ground
        line = Line(camera_point, rot_point)

        # find the intersection with the ground
        pt = line.plane_intersection(ground_plane, forward_only=True)
        if pt is None:
            # its pointing up into the sky
            return None

        (view_lat, view_lon) = mp_util.gps_offset(lat, lon, pt.y, pt.x)

        icon = self.mpstate.map.icon('camera-small-red.png')
        self.mpstate.map.add_object(mp_slipmap.SlipIcon('gimbalview',
                                                        (view_lat,view_lon),
                                                        icon, layer='GimbalView', rotation=0, follow=False))