Imageresponse: fix offset in FF.

[BLD-711](https://edx-wiki.atlassian.net/browse/BLD-711)

The reason of bug is in using [offsetX/offsetY properties](http://stackoverflow.com/questions/17293681/why-firefox-event-offsetx-and-offsety-are-undefined) that are implemented inconsistently across browsers.

@valera-rozuvan please review.