Expose main thread id

This will allow to assert that a function is called from the main
thread.

app/src/main.c

@@ -16,6 +16,7 @@
 #include "usb/scrcpy_otg.h"
 #include "util/log.h"
 #include "util/net.h"
+#include "util/thread.h"
 #include "version.h"
 
 #ifdef _WIN32
@@ -67,6 +68,9 @@ main_scrcpy(int argc, char *argv[]) {
         goto end;
     }
 
+    // The current thread is the main thread
+    SC_MAIN_THREAD_ID = sc_thread_get_id();
+
 #ifdef SCRCPY_LAVF_REQUIRES_REGISTER_ALL
     av_register_all();
 #endif

app/src/util/thread.c

@@ -6,6 +6,8 @@
 
 #include "log.h"
 
+sc_thread_id SC_MAIN_THREAD_ID;
+
 bool
 sc_thread_create(sc_thread *thread, sc_thread_fn fn, const char *name,
                  void *userdata) {

app/src/util/thread.h

@@ -39,6 +39,8 @@ typedef struct sc_cond {
     SDL_cond *cond;
 } sc_cond;
 
+extern sc_thread_id SC_MAIN_THREAD_ID;
+
 bool
 sc_thread_create(sc_thread *thread, sc_thread_fn fn, const char *name,
                  void *userdata);