Continue work on png writer.

src/core/file_utilities/File.cpp

@@ -32,9 +32,23 @@ std::ofstream* File::GetOutHandle() const
     return mOutHandle.get();
 }
 
-unsigned char File::readNextByte()
+std::optional<unsigned char> File::readNextByte()
 {
-    return mInHandle->get();
+    if (mInHandle->good())
+    {
+        if (auto val = mInHandle->get(); val == EOF)
+        {
+            return std::nullopt;
+        }
+        else
+        {
+            return val;
+        }
+    }
+    else
+    {
+        return std::nullopt;
+    }
 }
 
 void File::Open(bool asBinary)