Container padding gaps with percentPosition

**1** Gaps will occur when layout is changed — on resize, or item size changes — with `percentPosition: true` AND when the container element has padding. See demo: http://codepen.io/desandro/pen/ZGdqqZ

**2** Gaps in columns with percent width items and container padding. See demo http://codepen.io/desandro/pen/KpjGxr

Start by reproducing the two cases in the linked CodePen demos, including resize or item-size changes and percent-width columns with container padding. Trace the layout path handling percentPosition and padding; done means both demos no longer show gaps after relayout.

Written by the indexing model from the issue text.