Suggestions on improving rendering perf
Hi! As I pointed out on Twitter: “moving picker point doesn't feel smooth”, I would like to share with you some findings (sorry, I have no time to prepare a PR).
1. A huge perf loss is [here](https://github.com/casesandberg/react-color/blob/master/src/components/common/Saturation.jsx#L58-L61). That code means that on every mouse move it does query the DOM for some layout info, which is expensive operation. It's possible to reduce the frequency of that operations by caching those values within `componentDidMount` call and assigning the function which will update those props on `container` element resize (if it's resizeable of course) and window scroll, since these are cases when that props can change.
2. Moving the picker can be done with CSS Transforms, rather than [`top/left`](https://github.com/casesandberg/react-color/blob/master/src/components/common/Saturation.jsx#L40-L41) props. In addition to this `will-change: transform;` CSS rule can be added to let browser know and thus optimize frequently changing `transform` prop on that element.
3. Not actually related, but I didn't notice a single `shouldComponentUpdate` check :)

_Here's a short clip with the resulting perf I got after applying all of the suggestions from above: [see](https://www.dropbox.com/s/omqfvjchd58d6b8/Untitled.mov?dl=0). I know the picker is out of position :)_
