Don't force sliders to get evaluated on load

Rebased version of #1079

Since the load and play progress sliders are guaranteed to start from zero, set that through CSS. Calling Slider.prototype.update forces a re-flow because element dimensions are queried and style rules changed. That reflow consistently took around 60ms on my laptop which would mean dropped frames and "jerkiness" on initialization. Updated to handle scenarios where the video.js stylesheet is loaded after the `ready` event has already fired.