Pass event through to getMousePx and getMouseCoords. (Fixes: #37387)

Summary:
On Firefox, dragging the rectangle in the Visualizing Derivatives
exercise results in an "event is not defined" error.

Test Plan:
Visit /exercises/visualizing_derivatives.html with Firefox 13 and Chrome 24 on
Mac OS X.

Reviewers: stephanie

Reviewed By: stephanie

CC: tom, csilvers

Differential Revision: http://phabricator.khanacademy.org/D1016