Add <audio> and <video> player backends
These patches enables audio and video playing inside Servo.
It is bit hackish way to enable it, thus the purpose of this pull request is for an early request for comments.
It is tested with the current servo-media GStreamer backend in Linux.
~~The produced layout is not correct, since the elements after the video seems to be stacked behind, and the same with the scrolling bars.~~
~~There is no JavaScript interface yet~~, neither controls.
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Enable fake fillText API in nightly builds.
The [three.js examples](https://threejs.org/examples/) are really useful for checking Servo WebGL support, but if the fillText API is not available they only display a blank screen. I think it's worth lying to web content about this API in order to make it easy to make use of these examples without using local development builds.
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Reacting to environment changes
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- [x] These changes fix a subset of #11416
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