Mouse input is broken in all browsers

https://github.com/emscripten-ports/SDL2/commit/8d47621114eb254c33a35a44bfac39b3d7d4dca2

This commit breaks input handling in all browsers.
See emscripten's `fillMouseEventData`. Pay attention to this line.

```
var rect = (target === window) ? { left: 0, top: 0 } : target.getBoundingClientRect();
```

Now it's executed with `target == document`, with `getBoundingClientRect` undefined.
