Fix offsets for canvas

If canvas embedded in div with ```position: absolute``` then simple call to Module['canvas'].offset Top|Left not enough to get mouse position.

```
.canvas {
	position: absolute;
	left: 0;
	width: 100%;
	top: 43px;
	bottom: 0;
}

.canvas #canvas {
	position: absolute;
	left: 0;
	width: 100%;
	top: 0px;
	height: 100%;
}
//...

  <div class="canvas">
    <canvas id="canvas" oncontextmenu="event.preventDefault()">
  </div>
```

In the snippet call to Module['canvas'].offsetTop returns 0, but real offset is 43px. So i fixed the code, but maybe i should implement cache for offset.