Add resolutionUnit to metadata and as tiff option #3023

Co-authored-by: Lovell Fuller <github@lovell.info>

src/metadata.h

@@ -40,6 +40,7 @@ struct MetadataBaton {
   std::vector<int> delay;
   int pagePrimary;
   std::string compression;
+  std::string resolutionUnit;
   std::vector<std::pair<int, int>> levels;
   int subifds;
   std::vector<double> background;