Call my callback function in other thread with proxying.h

When I call my "**cb**" pointer function in the main thread, my "**cb_js**" function on the JS side is triggered.
However, when I do this in a separate thread, it does not work.

**C++:**
```
void (*cb)(int a);

extern "C" EMSCRIPTEN_KEEPALIVE void init_cb(void(*cb_js)(int a)){
cb = cb_js;
}

void process(int a){
cb(a);
}

extern "C" EMSCRIPTEN_KEEPALIVE void Start(){
process(5);
}
```

**JS:**
```
function cb_js(a){
console.log(a);
}

let ptr_cb = Module.addFunction(cb_js, "vi");
Module.ccall("init_cb", "void", ["number"], [ptr_cb]);
Module.ccall("Start", "void", [], []);
```

The codes above are working.
However, when **I want to access my callback function in a other thread** as follows, I cannot access it.
I think this is possible with "**proxying.h**".

**C++:**
```
extern "C" EMSCRIPTEN_KEEPALIVE void Start(){
std::thread([](){
process(5);
}).join();
```

}