QML MediaPlayer: allow video rendering to hardware layer

Currently, the QML media player can only render a video using a texture. There should also be a way to render the video directly into a hardware layer, with the QML scene on top of it.

This requires a new property in the QML API to select the rendering method, a new media control interface, and actual implementations in the plugins where it make sense (at least GStreamer, probably Android).

This also depends on the ability to punch a hole in the QML scene.