GTiff: for a paletted TIFF with nodata, set the alpha component of the color entry that matches the nodata value to 0, so as gdal_translate -expand rgba works properly

git-svn-id: https://svn.osgeo.org/gdal/trunk@28000 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx