Added function to allow for unicasting to specific IP address

[BlindElephants]

Added function to artnet.c/.h that allows for sending artnet messages to a specific IP address. This function otherwise replicates artnet_send_dmx, while stripping out unnecessary conditionals. The target IP address is provided as a uint32_t as the last argument to the new function. This value can be derived by using the inet_addr() function, as used elsewhere in libartnet and documented here.

int artnet_send_dmx_to_addr(artnet_node vn, int port_id, int16_t length, const uint8_t *data, uint32_t address) {
  node n = vn;
  artnet_packet_t p;
  input_port_t *port;

  check_nullnode(vn);

  if (n->state.mode != ARTNET_ON) {
    return ARTNET_EACTION;
  }
  if (port_id < 0 || port_id >= ARTNET_MAX_PORTS) {
    artnet_error("%s : port index out of bounds (%i < 0 || %i > ARTNET_MAX_PORTS)", __FUNCTION__, port_id);
    return ARTNET_EARG;
  }
  port = &n->ports.in[port_id];

  if (length < 1 || length > ARTNET_DMX_LENGTH) {
    artnet_error("%s : Length of dmx data out of bounds (%i < 1 || %i > ARTNET_MAX_DMX)", __FUNCTION__, length);
    return ARTNET_EARG;
  }

  if (port->port_status & PORT_STATUS_DISABLED_MASK) {
    artnet_error("%s : attempt to send on a disabled port (id:%i)", __FUNCTION__, port_id);
    return ARTNET_EARG;
  }

  // ok we're going to send now, make sure we turn the activity bit on
  port->port_status = port->port_status | PORT_STATUS_ACT_MASK;

  p.length = sizeof(artnet_dmx_t) - (ARTNET_DMX_LENGTH - length);

  // now build packet
  memcpy(&p.data.admx.id, ARTNET_STRING, ARTNET_STRING_SIZE);
  p.data.admx.opCode =  htols(ARTNET_DMX);
  p.data.admx.verH = 0;
  p.data.admx.ver = ARTNET_VERSION;
  p.data.admx.sequence = port->seq;
  p.data.admx.physical = port_id;
  p.data.admx.universe = htols(port->port_addr);

  // set length
  p.data.admx.lengthHi = short_get_high_byte(length);
  p.data.admx.length = short_get_low_byte(length);
  memcpy(&p.data.admx.data, data, length);

  p.to.s_addr = address;
  artnet_net_send(n, &p);
  return ARTNET_EOK;
}