shoes/shoes4 — pull request #843 by jasonrclark (2014-08-10 21:44:16+00:00)
Title: Fix crash computing height with hidden elements

Hidden elements don't receive an absolute_bottom from the slot positioning
logic, but computing the slot height wasn't taking into account that fact when
finding the maximum value to use.

This commit properly ignores elements that are hidden in the calculation. If
other nil's slip into the absolute_bottom list for reasons other than
visibility, those should be dealt with explicitly.

Fixes #796