TPS and Polynomial GCP transformations: recognize 'GCPZ' attribute of GCP, in addition to 'Z', since 'GCPZ' is what we currently generate. But GDAL 1.11/2.0 will generate 'Z' for consistency (#5326)

git-svn-id: https://svn.osgeo.org/gdal/branches/1.10@26732 f0d54148-0727-0410-94bb-9a71ac55c965