Replace SDL_net by custom implementation

SDL_net is not very suitable for scrcpy.

For example, SDLNet_TCP_Accept() is non-blocking, so we have to wrap it
by calling many SDL_Net-specific functions to make it blocking.

But above all, SDLNet_TCP_Open() is a server socket only when no IP is
provided; otherwise, it's a client socket. Therefore, it is not possible
to create a server socket bound to localhost, so it accepts connections
from anywhere.

This is a problem for scrcpy, because on start, the application listens
for nearly 1 second until it accepts the first connection, supposedly
from the device. If someone on the local network manages to connect to
the server socket first, then they can stream arbitrary H.264 video.
This may be troublesome, for example during a public presentation ;-)

Provide our own simplified API (net.h) instead, implemented for the
different platforms.

app/src/scrcpy.c

@@ -6,7 +6,6 @@
 #include <libavformat/avformat.h>
 #include <sys/time.h>
 #include <SDL2/SDL.h>
-#include <SDL2/SDL_net.h>
 
 #include "command.h"
 #include "common.h"
@@ -19,7 +18,7 @@
 #include "inputmanager.h"
 #include "log.h"
 #include "lockutil.h"
-#include "netutil.h"
+#include "net.h"
 #include "screen.h"
 #include "server.h"
 #include "tinyxpm.h"
@@ -104,8 +103,8 @@ SDL_bool scrcpy(const char *serial, Uint16 local_port, Uint16 max_size, Uint32 b
     // managed by the event loop. This blocking call blocks the event loop, so
     // timeout the connection not to block indefinitely in case of SIGTERM.
 #define SERVER_CONNECT_TIMEOUT_MS 2000
-    TCPsocket device_socket = server_connect_to(&server, serial, SERVER_CONNECT_TIMEOUT_MS);
-    if (!device_socket) {
+    socket_t device_socket = server_connect_to(&server, serial, SERVER_CONNECT_TIMEOUT_MS);
+    if (device_socket == INVALID_SOCKET) {
         server_stop(&server, serial);
         ret = SDL_FALSE;
         goto finally_destroy_server;