emscripten: Restore a hack to asyncify send_agsevent() callers

It was inadvertently removed in commit 61ad428.

src/CMakeLists.txt

@@ -162,6 +162,9 @@ if (EMSCRIPTEN)
     -sASYNCIFY_IGNORE_INDIRECT=1
     -sASYNCIFY_REMOVE=SDL_Delay
     -sASYNCIFY_IMPORTS=wait_vsync
+    # Functions that call an asynchronous function via a function pointer must
+    # be added to ASYNCIFY_ADD, even if it had a direct call to another
+    # asynchronous function, so that the indirect call itself is instrumented.
     -sASYNCIFY_ADD=commands2F60,nact_main,send_agsevent,cb_waitkey_sprite
     -sALLOW_MEMORY_GROWTH=1
     -sNO_EXIT_RUNTIME=1

src/sdl_event.c

@@ -222,6 +222,13 @@ void send_agsevent(enum agsevent_type type, int code) {
 		.mousey = mousey
 	};
 	nact->ags.eventcb(&agse);  // Async in emscripten
+
+#ifdef __EMSCRIPTEN__
+	// HACK: this ensures that callers of this function are instrumented.
+	// This is necessary because ASYNCIFY_PROPAGATE_ADD does not work:
+	// https://github.com/emscripten-core/emscripten/issues/23015
+	emscripten_sleep(0);
+#endif
 }
 
 void sdl_setCursorInternalLocation(int x, int y) {