respect the selected port if sending over loopback

`uv -t testcard -d gl -P 5004` didn't select the port 5004 until now

Author: MartinPulec

src/main.cpp

@@ -560,7 +560,7 @@ struct ug_options {
 
         const char *display_cfg = "";
         const char *requested_video_fec = "none";
-        int port_base = PORT_BASE;
+        int port_base = -1;
         int requested_ttl = -1;
         int video_rx_port = -1, video_tx_port = -1;
 
@@ -1216,16 +1216,19 @@ static int adjust_params(struct ug_options *opt) {
                 opt->video_rxtx_mode |= MODE_SENDER;
         }
 
-        // use dyn ports if sending only to ourselves or neither sending nor receiving
+        // use dyn ports if unset, sending only to ourselves or neither sending nor receiving
         const unsigned mode_both = MODE_RECEIVER | MODE_SENDER;
         if (is_host_loopback(opt->requested_receiver) && ((opt->video_rxtx_mode & mode_both) == mode_both || (opt->video_rxtx_mode & mode_both) == 0)
-                        && (opt->video_rx_port == -1 && opt->video_tx_port == -1)) {
+                        && (opt->video_rx_port == -1 && opt->video_tx_port == -1 && opt->port_base == -1)) {
                 opt->video_rx_port = opt->video_tx_port = 0;
         }
         if (is_host_loopback(opt->requested_receiver) && ((audio_rxtx_mode & mode_both) == mode_both || (audio_rxtx_mode & mode_both) == 0)
-                        && (opt->audio.recv_port == -1 && opt->audio.send_port == -1)) {
+                        && (opt->audio.recv_port == -1 && opt->audio.send_port == -1 && opt->port_base == -1)) {
                 opt->audio.recv_port = opt->audio.send_port = 0;
         }
+        if (opt->port_base == -1) {
+                opt->port_base = PORT_BASE;
+        };
 
         if (opt->video_rx_port == -1) {
                 if ((opt->video_rxtx_mode & MODE_RECEIVER) == 0U) {