Add --help

Provide command-line help, with -h/--help option.

app/src/main.c

@@ -10,19 +10,59 @@
 
 struct args {
     const char *serial;
+    SDL_bool help;
     Uint16 port;
     Uint16 max_size;
 };
 
+static void usage(const char *arg0) {
+    fprintf(stderr,
+        "Usage: %s [options] [serial]\n"
+        "\n"
+        "    serial\n"
+        "        The device serial number. Mandatory only if several devices\n"
+        "        are connected to adb.\n"
+        "\n"
+        "Options:\n"
+        "\n"
+        "    -h, --help\n"
+        "        Print this help.\n"
+        "\n"
+        "    -m, --max-size value\n"
+        "        Limit both the width and height of the video to value. The\n"
+        "        other dimension is computed so that the device aspect-ratio\n"
+        "        is preserved.\n"
+        "        Default is %d%s.\n"
+        "\n"
+        "    -p, --port port\n"
+        "        Set the TCP port the client listens on.\n"
+        "        Default is %d.\n"
+        "\n"
+        "Shortcuts:\n"
+        "\n"
+        "    Ctrl+f: switch fullscreen mode\n"
+        "    Ctrl+g: resize window to 1:1 (pixel-perfect)\n"
+        "    Ctrl+x: resize window to optimal size (remove black borders)\n"
+        "\n",
+        arg0,
+        DEFAULT_MAX_SIZE, DEFAULT_MAX_SIZE ? "" : " (unlimited)",
+        DEFAULT_LOCAL_PORT);
+}
+
 static int parse_args(struct args *args, int argc, char *argv[]) {
     static const struct option long_options[] = {
+        {"help",     no_argument,       NULL, 'h'},
         {"port",     required_argument, NULL, 'p'},
         {"max-size", required_argument, NULL, 'm'},
         {NULL,       0,                 NULL, 0  },
     };
     int c;
-    while ((c = getopt_long(argc, argv, "p:m:", long_options, NULL)) != -1) {
+    while ((c = getopt_long(argc, argv, "hp:m:", long_options, NULL)) != -1) {
         switch (c) {
+            case 'h': {
+                args->help = SDL_TRUE;
+                break;
+            }
             case 'p': {
                 char *endptr;
                 long value = strtol(optarg, &endptr, 0);
@@ -72,14 +112,21 @@ int main(int argc, char *argv[]) {
     int res;
 
     struct args args = {
+        .help = SDL_FALSE,
         .serial = NULL,
         .max_size = DEFAULT_MAX_SIZE,
         .port = DEFAULT_LOCAL_PORT,
     };
     if (parse_args(&args, argc, argv)) {
+        usage(argv[0]);
         return 1;
     }
 
+    if (args.help) {
+        usage(argv[0]);
+        return 0;
+    }
+
     av_register_all();
 
     if (avformat_network_init()) {