[cairo] [PATCH] Fix fixup_unbounded_boxes() to work even if the box is smaller than the extents

[Jeff Muizelaar]

This patch fixes the problem we ran into here:
https://bugzilla.mozilla.org/show_bug.cgi?id=668921

Currently fixup_unbounded_boxes assumes that the extents are tight when 
we only have one box. This is not always true. e.g. when we tesselate boxes
we can end up with only one box which is smaller than the extents and 
the extents are not updated.

Does this look like the correct fix?

-Jeff