Wrap SDL thread functions into scrcpy-specific API

The goal is to expose a consistent API for system tools, and paves the
way to make the "core" independant of SDL in the future.

app/src/receiver.h

@@ -4,17 +4,16 @@
 #include "common.h"
 
 #include <stdbool.h>
-#include <SDL2/SDL_mutex.h>
-#include <SDL2/SDL_thread.h>
 
 #include "util/net.h"
+#include "util/thread.h"
 
 // receive events from the device
 // managed by the controller
 struct receiver {
     socket_t control_socket;
-    SDL_Thread *thread;
-    SDL_mutex *mutex;
+    sc_thread thread;
+    sc_mutex mutex;
 };
 
 bool