Fix for unbounded height crash on centering

With certain values, a centered text segment is coerced into thinking it
has unbounded height (which is set as a symbol, not a number) and was
later incorrectly used in some position math.

Calculating the first_height now gates for the :unbounded symbol so we
don't crash on it.

Fixes #824
Merge pull request #842 from shoes/centering_crash

Fix for unbounded height crash on centering

Changelog: bugfix