Rename screen -> scrcpy -> main

Rename scrcpy.c to main.c (this file handles the command line parsing),
and screen.c to scrcpy.c (it exposes the entry point scrcpy()).

app/src/main.c

@@ -0,0 +1,73 @@
+#include "scrcpy.h"
+
+#include <unistd.h>
+#include <libavformat/avformat.h>
+#include <SDL2/SDL.h>
+
+#define DEFAULT_LOCAL_PORT 27183
+
+struct args {
+    const char *serial;
+    Uint16 port;
+};
+
+int parse_args(struct args *args, int argc, char *argv[]) {
+    int c;
+    while ((c = getopt(argc, argv, "p:")) != -1) {
+        switch (c) {
+        case 'p': {
+            char *endptr;
+            long int value = strtol(optarg, &endptr, 0);
+            if (*optarg == '\0' || *endptr != '\0') {
+                SDL_LogError(SDL_LOG_CATEGORY_APPLICATION, "Invalid port: %s\n", optarg);
+                return -1;
+            }
+            if (value & ~0xffff) {
+                SDL_LogError(SDL_LOG_CATEGORY_APPLICATION, "Port out of range: %ld\n", value);
+                return -1;
+            }
+            args->port = (Uint16) value;
+            break;
+        }
+        default:
+            // getopt prints the error message on stderr
+            return -1;
+        }
+    }
+
+    int index = optind;
+    if (index < argc) {
+        args->serial = argv[index++];
+    }
+    if (index < argc) {
+        SDL_LogError(SDL_LOG_CATEGORY_APPLICATION, "Unexpected additional argument: %s\n", argv[index]);
+        return -1;
+    }
+    return 0;
+}
+
+int main(int argc, char *argv[]) {
+    int res;
+
+    struct args args = {
+        .serial = NULL,
+        .port = DEFAULT_LOCAL_PORT,
+    };
+    if (parse_args(&args, argc, argv)) {
+        return 1;
+    }
+
+    av_register_all();
+
+    if (avformat_network_init()) {
+        return 1;
+    }
+
+    SDL_LogSetAllPriority(SDL_LOG_PRIORITY_DEBUG);
+
+    res = scrcpy(args.serial, args.port) ? 0 : 1;
+
+    avformat_network_deinit(); // ignore failure
+
+    return res;
+}