modify GDALGCPsToGeotransform() to do the regression in normalized coordinates to make the math more stable.  Expose new GDALComposeGeoTransforms() function.

git-svn-id: https://svn.osgeo.org/gdal/trunk@26603 f0d54148-0727-0410-94bb-9a71ac55c965
test GDALGCPsToGeoTransform()

git-svn-id: https://svn.osgeo.org/gdal/trunk@26604 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx