Image cropper demo errors (patch included)

When using the code provided in the Image Cropper demo, under certain conditions, it can cause errors in the callback functions that are provided. This can happen if the cropper is placed out of bounds, or under other odd circumstances.

The problem is in the code that inverts the values and adjusts the background of the resizable. If a value is already negative, it causes a double negative, and is not parsed.

Fix: replace all instances of

this.style.backgroundPosition = '-' + (self.position.left) + 'px -' + (self.position.top) + 'px';

with

this.style.backgroundPosition = (self.position.left * -1) + 'px ' + (self.position.top * -1) + 'px';

Seems like a more logical way to invert a value, and won't result in a double negative.