Only track viewing time not on current slide.
Only track viewing time not on current slide.

Previously this would track all time spent on slides. This patch makes
it so that only time where the student is looking at a slide other than
what the presenter is on is recorded.

berkes/showoff,slambert/showoff,puppetlabs/showoff,wanix/showoff,wanix/showoff,berkes/showoff,slambert/showoff,puppetlabs/showoff,puppetlabs/showoff,kublaj/showoff,nomadicoder/showoff,kublaj/showoff,kublaj/showoff,marrero984/showoff,binford2k/showoff,binford2k/showoff,marrero984/showoff,nomadicoder/showoff,berkes/showoff,puppetlabs/showoff,slambert/showoff,slambert/showoff,binford2k/showoff,kublaj/showoff,kublaj/showoff,marrero984/showoff,nomadicoder/showoff,wanix/showoff,nomadicoder/showoff,berkes/showoff,berkes/showoff,binford2k/showoff,marrero984/showoff