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
Merge pull request #843 from shoes/crash_compute_content_height

Fix crash computing height with hidden elements

* Changelog: bugfix