review and clean-up of the CSS 3D GPU-accelerated code routines (now off by default)

This is a follow-up to:
https://github.com/readium/SDKLauncher-OSX/issues/16
https://github.com/readium/readium-shared-js/pull/132

GPU-accelerated CSS rendering was creating too many nasty side effects (whilst only fixing one particular type of bug), and the native WebView can be configured so that surface compositing works well with the iframes inside reader.html.