Continue work on png writer.

src/core/file_utilities/File.h

@@ -7,6 +7,7 @@
 #include <fstream>
 #include <memory>
 #include <vector>
+#include <optional>
 
 using Path = std::filesystem::path;
 
@@ -48,7 +49,7 @@ public:
 
     void Close();
 
-    unsigned char readNextByte();
+    std::optional<unsigned char> readNextByte();
 
 
 private: