Event handlers never consume events in Asyncify

Per html5.h, event handlers can return TRUE to consume the event, i.e., `event.preventDefault()` is called. This does not work in Asyncify.

Here's a [simple test case](https://gist.github.com/devappd/1895eba742197d4affdee03a6e6eff48) for mouse events:

* [Normal build](https://devappd.github.io/tests/asyncify-events/main.html)
* [Asyncify build](https://devappd.github.io/tests/asyncify-events/main-asyncify.html)

The expected result is that the default handler is never invoked on the canvas, e.g., mouse clicks do not cause the canvas to gain focus.

I saw that the event handlers check the result of `dynCall_iiii()` to determine whether to call `e.preventDefault()`. This result is never passed in Asyncify.

Therefore, I get the desired result if I modify the compiled Module as below:

```diff
function registerMouseEventCallback(target, userData, useCapture, callbackfunc, eventTypeId, eventTypeString, targetThread) {
/* ... */

var mouseEventHandlerFunc = function(ev) {
var e = ev || event;

// TODO: Make this access thread safe, or this could update live while app is reading it.
fillMouseEventData(JSEvents.mouseEvent, e, target);

- if ((function(a1, a2, a3) { dynCall_iiii.apply(null, [callbackfunc, a1, a2, a3]); })(eventTypeId, JSEvents.mouseEvent, userData)) e.preventDefault();
+ if ((function(a1, a2, a3) { return dynCall_iiii.apply(null, [callbackfunc, a1, a2, a3]); })(eventTypeId, JSEvents.mouseEvent, userData)) e.preventDefault();
};

/* ... */
}
```

See [Asyncify build with the fix applied](https://devappd.github.io/tests/asyncify-events/main-asyncify-fixed.html), which passes the test.

Is this fix appropriate? If so, I can use guidance on patching the codebase as I'm unfamiliar with Asyncify modifications.