Bug1100528-[Gallery[RTL]-Mirror slider thumb to progress left while playing video in gallery r=djf

I think these would be better if you put them right next to the ltr styles around line 130, so readers can see both ltr and rtl presentations in the same place.

An alternative here would be to use flexbox layout on .videoPlayerSlider so that the elapsed, progress and duration elements are laid out ltr or rtl automatically. But this is easier for now. We can refactor if and when we convert this to a web component.