Do proper x velocity comparison when pan ends

This partially address my previous pull request (https://github.com/edgecase/ECSlidingViewController/pull/153) and makes anchor-to-left panning behaviour consistent with anchor-to-right behaviour.

Before, revealing the under-right view would usually anchor-to-left, even with slight rightward velocity.

Now, for both directions, the sliding view will tend to reset unless the velocity is great enough toward the anchor.

(The bug where it resets when dragged slowly past the anchor point is still present, but I'll try to fix it in another pull request)