autoplay: When setting new timeout, clear the old one.
autoplay: When setting new timeout, clear the old one.

Autoplay plugin sets its next timeout on impress:stepenter. However,
if you manually step through multiple slides quickly, these queue
up and then start firing in rapid succession. This fix adds a
clearTimeout() before setting a new one, so that there's only
at most one timeout queued at any given time.

henrikingo/impress.js,henrikingo/impress.js