Fixed wrong plugin options behaviour

While working on MR !230 I noticed that the options read I entered into several plugins could not be read after reading the image.

**The patch fixes problems reading options in plugins and adds option checking in the readtest.cpp.**

In particular, the reading test does the following additional actions:
- reads options before reading the image;
- compare the options read with the options returned by the reader after reading the image;
- compares the format and size of the returned image with the format and size returned by the reader.

src/imageformats/hdr_p.h

@@ -22,6 +22,13 @@ public:
     QVariant option(QImageIOHandler::ImageOption option) const override;
 
     static bool canRead(QIODevice *device);
+
+private:
+    /*!
+     * \brief m_imageSize
+     * Image size cache used by option()
+     */
+    QSize m_imageSize;
 };
 
 class HDRPlugin : public QImageIOPlugin