Added possible fix for issue ericdrowell/KineticJS#884.

This change seems to resolve the issue described in #884. I have tested out a few scenarios with various `pixelRatio` values, with different clipping values, and with the large application that I'm working on. So far, I have not seen any side-effects.

The change avoids calling `Context.reset()` after drawing the clipping area:
```
 reset: function() {
    var pixelRatio = this.getCanvas().getPixelRatio();
    this.setTransform(1 * pixelRatio, 0, 0, 1 * pixelRatio, 0, 0);
}
```

Perhaps there is a deeper issue, where the values sent to `setTransform()` behave differently between the "display" canvas and the "hit" canvas.