[whatwg] Reporting mid-stream resolution change on <video>

[Ami Fischman]

Today the <video> tag exposes video{Width,Height} attributes that allow the
page to discover the resolution of the playing media once metadataloaded
has fired.  However there is no way for the page to find out that the media
resolution has changed mid-stream (short of polling the tag constantly,
which is obviously unappealing).

Examples where mid-stream resolution change can happen include:
- WebRTC up/down-scaling a video stream at the source to adhere to
available bandwidth or other requirements
- MSE shifting between quality levels of source material
- Static/VOD/classic video files that change resolution mid-stream b/c of
different source material being concatenated together.

One way to enable this would be to add a "metadatachange" event to the list
of media elements events (
http://www.whatwg.org/specs/web-apps/current-work/multipage/the-video-element.html#event-definitions
).

(this has come up recently in a public-webrtc
thread<http://lists.w3.org/Archives/Public/public-webrtc/2013Dec/0040.html>,
and previously in private discussions)

Cheers,
-a