* We now trigger `sourceset` any time a `<source>` element is appended to a mediaEl with no source.
* `load` should always fire a `sourceset`
* `sourceset` should always be the absolute url.
In Chrome/Firefox/Safari appending a <source> element when the media element has no source, causes what we think of as a `sourceset`. These changes make our code actual fire that event.
This was found due to the work done in #4660. Basically we reload the video element twice on every source with preload set to auto. This can potentially cause the same data to be downloaded twice.
If .load() is called, we don't know what the new source will be because of the asynchronous nature of the resource selection algorithm. However, we can know if the src attribute is set on the video element because that's what will be chosen. So, set the src property on the event object to be the src attribute or be the empty string. This matches how currentSrc works where it is an empty string when no source is set. So, when sourceset is triggered and src is "" in the event object, it means that the source is changing.
In addition, we're going to be releasing sourceset as an experimental feature so we could improve the API and make changes in minor releases.
Trigger a sourceset event whenever the source is set in the Html5 tech, including initial source. We override the video element's src and setAttribute methods so that we can trigger the sourceset event when people change the src with both the video element and our API methods.
The event object for sourceset will contain the src string that was provided at the time the sourceset was triggered. This is mostly important if a source is being set while a tech is changing.
A Tech has a featuresSourceset option that it can set to for sourceset handling. It can then call the helper triggerSourceset(src) to trigger the sourceset.
