[box-layout] Adjust amount we need to expand for padding
[box-layout] Adjust amount we need to expand for padding

Based on patch from: Owen W. Taylor <otaylor@fishsoup.net>

When we determine the amount of extra space to expand through
as avail_width - pref_width, we need to adjust both of the elements
for the padding, not just avail_width.

Fixes: http://bugzilla.moblin.org/show_bug.cgi?id=6308

jonnylamb/mx,clutter-project/mx,pwithnall/mx,jonnylamb/mx,jonnylamb/mx,clutter-project/mx,pwithnall/mx,clutter-project/mx,jonnylamb/mx,clutter-project/mx,pwithnall/mx,pwithnall/mx