Video/audio layer looping

[clabe45]

For a layer to loop, it must have a duration of Infinity and the content must repeat endlessly. Both video and audio layers extend the AudioSourceMixin layer mixin, which implements all media state and functionality of these two layers (start, stop, duration, etc). The task is to add a loop option for this mixin and configure the layer to loop endlessly. The audio and video layers will also need to accept this new option and pass it down to the mixin constructor.