imagedata: report if file is DNG

rtengine/imagedata.h

@@ -59,6 +59,7 @@ private:
     time_t modTimeStamp;
     bool isPixelShift;
     bool isHDR;
+    bool isDNG;
     int w_;
     int h_;
 
@@ -69,6 +70,7 @@ public:
     unsigned int getFrameCount() const override;
     bool getPixelShift() const override;
     bool getHDR() const override;
+    bool getDNG() const override;
     std::string getImageType() const override;
     IIOSampleFormat getSampleFormat() const override;
     bool hasExif() const override;