ras: code revamped

- Progressive load from file
- Added support for 1-bit image
- Images with palette are now Index (instead of RGB32)
- Added options support (Size, Format)
- Added some test cases
- Improved performance by directly accessing the scanline
- Support for more RAS extension (taken from GIMP)

The code should works "as is" also on KF5.

src/imageformats/ras_p.h

@@ -19,6 +19,9 @@ public:
     bool canRead() const override;
     bool read(QImage *image) override;
 
+    bool supportsOption(QImageIOHandler::ImageOption option) const override;
+    QVariant option(QImageIOHandler::ImageOption option) const override;
+
     static bool canRead(QIODevice *device);
 };