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/stream.h

@@ -7,15 +7,15 @@
 #include <stdint.h>
 #include <libavformat/avformat.h>
 #include <SDL2/SDL_atomic.h>
-#include <SDL2/SDL_thread.h>
 
 #include "util/net.h"
+#include "util/thread.h"
 
 struct video_buffer;
 
 struct stream {
     socket_t socket;
-    SDL_Thread *thread;
+    sc_thread thread;
     struct decoder *decoder;
     struct recorder *recorder;
     AVCodecContext *codec_ctx;