PDF writing: automatically adjust DPI in case the page dimension exceeds the 14400 maximum value (in user units) allowed by Acrobat (#5412)

git-svn-id: https://svn.osgeo.org/gdal/trunk@27026 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx