The layout node, and therefore the painting box, is frequently destroyed
and recreated. This causes us to forget the cached mouse position we use
to highlight media controls. Move this cached position to the DOM node
instead, which survives relayout.
If the video element has a 'controls' attribute, we now paint some basic
video controls over the video element. If no frame has been decoded yet,
we paint a play button on the center of the element.
If a frame has been decoded, we paint that frame and paint a control bar
on the bottom of the frame. This control bar currently only contains a
play/pause button, depending on the video's playback state. We will only
paint the control bar if the video is paused or hovered.
