Make it work over tcpip

"adb reverse" currently does not work over tcpip (i.e. on a device
connected by "adb connect"):
<https://issuetracker.google.com/issues/37066218>

To work around the problem, if the call to "adb reverse" fails, then
fallback to "adb forward", and reverse the client/server roles.

Keep the "adb reverse" mode as the default because it does not involve
connection retries: when using "adb forward", the client must try to
connect successively until the server listens.

Due to the tunnel, every connect() will succeed, so the client must
attempt to read() to detect a connection failure. For this purpose, when
using the "adb forward" mode, the server initially writes a dummy byte,
read by the client.

Fixes <https://github.com/Genymobile/scrcpy/issues/5>.

app/src/server.h

@@ -7,9 +7,11 @@
 struct server {
     const char *serial;
     process_t process;
-    socket_t server_socket;
+    socket_t server_socket; // only used if !tunnel_forward
     socket_t device_socket;
-    SDL_bool adb_reverse_enabled;
+    Uint16 local_port;
+    SDL_bool tunnel_enabled;
+    SDL_bool tunnel_forward; // use "adb forward" instead of "adb reverse"
     SDL_bool server_copied_to_device;
 };
 
@@ -18,7 +20,9 @@ struct server {
     .process = PROCESS_NONE,              \
     .server_socket = INVALID_SOCKET,      \
     .device_socket = INVALID_SOCKET,      \
-    .adb_reverse_enabled = SDL_FALSE,     \
+    .local_port = 0,                      \
+    .tunnel_enabled = SDL_FALSE,          \
+    .tunnel_forward = SDL_FALSE,          \
     .server_copied_to_device = SDL_FALSE, \
 }