Fix scrolling coordinates

[evindor]

These coordinates represent the scroll event location. Before this fix if you had NERDTree open (on the left) and was trying to scroll the document, only the NERDTree split scrolled. After the fix the split you have your mouse over is scrolled.

[coolwanglu]

Indeed, so the parameters is the location instead of the delta. However, probably we need to send multiple scrollwheel events if the delta is large.

[coolwanglu]

Thanks for the PR!