Render diagrams when slides are shown
Render diagrams when slides are shown

When the slideshow is started, the slide size is indeterminate. This
causes Mermaid to not know the correct SVG size, so it renders them
virtually invisible.

Doing it as the slide is shown means that the size is known.

marrero984/showoff,binford2k/showoff,puppetlabs/showoff,slambert/showoff,marrero984/showoff,marrero984/showoff,binford2k/showoff,slambert/showoff,puppetlabs/showoff,binford2k/showoff,slambert/showoff,binford2k/showoff,puppetlabs/showoff,puppetlabs/showoff,slambert/showoff,marrero984/showoff