Continue png writing.

src/image/png/PngWriter.h

@@ -2,6 +2,7 @@
 
 #include "PngHeader.h"
 #include "Image.h"
+#include "DeflateElements.h"
 
 #include <memory>
 #include <string>
@@ -46,6 +47,8 @@ private:
     unsigned mPngInfoUserSet{false};
     PngInfo mPngInfo;
     PngHeader mPngHeader;
+
+    Deflate::CompressionMethod mCompressionMethod{Deflate::CompressionMethod::NONE};
 };
 
 using PngWriterPtr = std::unique_ptr<PngWriter>;