Fixed iPad issue where you cannot switch between a full-screen image and video

Hi,

You cannot interact with a hidden HTML5 video in a callback on iPad, even if you attempt to show the video at the beginning of the callback code.

A pseudo-example code would be:
`````
$("#button").click(function () { $("video").show(); $(video).get(0).play(); }
````

This would not work. This PR fixes this by changing the behaviour to hide a video or poster using opacity, which means you can still interact with it even if it's not visible, or about to be made visible.