GTiff: add DISCARD_LSB creation option (lossy compression) to be best used with PREDICTOR=2 and LZW/DEFLATE compression

git-svn-id: https://svn.osgeo.org/gdal/trunk@27894 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx