v0.15.0: Release 0.15.0: video clips in a scene.

Video clips in a scene, behind the video feature.

Added

• Video::new().file("assets/demo.mp4") shows a clip as a node, sized
  with .width(pct(70)) like an image, with .radius(24.0) corners,
  effects and the camera. demo.play(), demo.pause() and
  demo.seek(2.0) control it; demo.playback_rate() and demo.time()
  are signals, so demo.playback_rate().to(0.25, 0.5) slows it down.
• The frame shown is a function of the scene's time, so a render and a
  seek show the same frame. Frames are decoded with ffmpeg and cached;
  WebM (VP9) and ProRes 4444 keep their alpha.
• The clip's sound goes to the mixer while it plays at rate 1;
  .muted(true) leaves it out.
• check reports video_unavailable when a clip cannot be opened.
• The video example.

Fixed

• The mixer read the first track of a file; for a video that is the
  picture. It now takes the first audio track.