Fix for text nodes causing erroneous svg group bounding boxes.

Text(Span) nodes report a bounding box of [0,0,0,0], which caused
legitimate bounding boxes to be incorrectly expanded to encompass
the origin.
We now check that each child has a non-zero bounding area before
using it to set or expand the group's bounding box.
Happy Easter.
Fix cairo header path in GoodMorning sample.