Workaround continuous resizing on Windows/MacOS

On Windows and MacOS, resizing blocks the event loop, so resizing events
are not triggered:
 - <https://bugzilla.libsdl.org/show_bug.cgi?id=2077>
 - <https://stackoverflow.com/a/40693139/1987178>

As a workaround, register an event watcher to render the screen from
another thread.

Since the whole event loop is blocked during resizing, the screen
content is not refreshed (on Windows and MacOS) until resizing ends.

app/src/scrcpy.c

@@ -35,7 +35,29 @@ static struct input_manager input_manager = {
     .screen = &screen,
 };
 
+#if defined(__APPLE__) || defined(__WINDOWS__)
+# define CONTINUOUS_RESIZING_WORKAROUND
+#endif
+
+#ifdef CONTINUOUS_RESIZING_WORKAROUND
+// On Windows and MacOS, resizing blocks the event loop, so resizing events are
+// not triggered. As a workaround, handle them in an event handler.
+//
+// <https://bugzilla.libsdl.org/show_bug.cgi?id=2077>
+// <https://stackoverflow.com/a/40693139/1987178>
+static int event_watcher(void* data, SDL_Event* event) {
+    if (event->type == SDL_WINDOWEVENT && event->window.event == SDL_WINDOWEVENT_RESIZED) {
+        // called from another thread, not very safe, but it's a workaround!
+        screen_render(&screen);
+    }
+    return 0;
+}
+#endif
+
 static void event_loop(void) {
+#ifdef CONTINUOUS_RESIZING_WORKAROUND
+    SDL_AddEventWatch(event_watcher, screen.window);
+#endif
     SDL_Event event;
     while (SDL_WaitEvent(&event)) {
         switch (event.type) {