IFF: add support for RGBN/RGB8 image data and CAT chunk

Add the following features:
- RGB8 image data support (test case added)
- RGBN image data support ([Clouds.iff](/uploads/9db869350f74421bf1813fa7d4332f4f/Clouds.iff))
- CAT chunk support: you can have more than one image for file (test case added)
- Image transformation support via EXIF data

[RGBN/RGB8](https://wiki.amigaos.net/wiki/RGBN_and_RGB8_IFF_Image_Data) files are used in Impulse's Turbo Silver and Imagine.

Closes #34

src/imageformats/iff_p.h

@@ -23,6 +23,11 @@ public:
     bool supportsOption(QImageIOHandler::ImageOption option) const override;
     QVariant option(QImageIOHandler::ImageOption option) const override;
 
+    bool jumpToNextImage() override;
+    bool jumpToImage(int imageNumber) override;
+    int imageCount() const override;
+    int currentImageNumber() const override;
+
     static bool canRead(QIODevice *device);
 
 private: