Fix camera problem in TestXMLUnstructuredGridReader.

Calling Elevation(90) causes the view plane normal to become
parallel to the "view up" vector, and subsequent orthogonalization
of the "view up" vector has an undefined result (i.e. it could
go either way, depending on the roundoff error on a particular
architecture).  The ViewUp should be set explicitly.

Change-Id: Ib22cf101ebb3b5d620e14d1d4cb13c25847c32ad
Merge topic 'ugridreader-test-fix' into master

d8229dfd Fix camera problem in TestXMLUnstructuredGridReader.