This relies on specific structure of interface{} being composed of two
gpointer's. The code expects something along the lines of:

```
/*
void cb_proxy(void* v, void* v2);
*/
import "C"

//export cb_proxy
func cb_proxy(v unsafe.Pointer, v2 unsafe.Pointer) {
    println("in cb_proxy")
    el := (*C.GstElement)(v)
    cb := (*gst.Element)(v2)
    mycall(el, cb)
}
func mycall(webrtc *C.GstElement, user_data *gst.Element) {
}
...
func main{
    ...
    source.SetCallback("on-negotiation-needed", C.cb_proxy)
    ...
}
```