Fixed a bug in mouseOffset() with Firefox, which lacks event.offsetX/Y properties

This commit addresses the issue with ggvis interactivity in Firefox, mostly with mouse clicking and dragging.
The current internal function mouseOffset() uses event.offsetX/Y which are undefined in Firefox, causing link_brush() to function abnormally. 

This commit should fix that.
Should be `===` instead of `==`.
Also, can you add a comment above this line explaining why it's necessary? Similar to the code in Shiny that Yihui provided a link to.