Fixed mouse position in Firefox.

Using Firefox 33 under Ubuntu 14.04 I couldn't get the script to detect my mouse position or "push" new particles in the animation when `interactivity.detect_on = "canvas"` (`"window"` is fine).
It seemed the fix from 88d5f02231bcc2e12f52d6d5755554058c3456cf broke it and ff preferes `e.clientX` and `e.clientY`.

I just added a "or" condition to let the browser decide what's best for him. I couldn't check the result under Chrome, Safari or IE but I guess that does the job.