Fix some sizing issues with video embeds, and improve video/audio embed shortcode flexibility.

* `loop`, `autoplay`, and `preload` are now available via the shortcode. Use them non-annoyingly, please!
* Attributes that pass through the filters are now proper key/value pairs, not an array of `key="value"` strings.
* `preload` defaults to `metadata` for videos. This fixes the vertical video preview and Safari ogv/webm playback issues.
* Wrap a div around video embeds to combat a ME.js issue with responsive width=100% themes. Props kovshenin.

Fixes #24134, #24798.

git-svn-id: https://core.svn.wordpress.org/trunk@24789 1a063a9b-81f0-0310-95a4-ce76da25c4cd