Don't use getScrollingParent if getContainer is a prop

Hi,

I've combined `react-sortable-hoc` with [simplebar-react](https://github.com/Grsmto/simplebar/tree/master/packages/simplebar-react). That was no big deal except one side effect: The autoscroll didn't worked anymore. So I've added `getContainer` to the props and returned the _correct_ element but it still didn't worked.

After investigating around I've found the solution: `getScrollingParent` was the bad guy. It bubbled to the completely wrong element because the element returned in `getContainer` has no `overflow` css property. But in my case it doesn't matter.

Now I've added this to the element, but I would suggest that if somebody provide a scroll container via `getContainer` the programmer knows what he/she is doing. My idea is to bypass `getScrollingParent` if `getContainer` exists.

BTW: `getScrollingParent` could return wrong elements, in example if the element has `overflow-x: auto;` but the sortable axis is `y`.

Trace the getContainer and getScrollingParent entry points, then follow how the selected element is used by autoscroll. Confirm the behavior with a supplied container and with an axis whose overflow does not match; done means getContainer takes precedence and the reported axis-specific case is handled.

Written by the indexing model from the issue text.