RGB: added options support

- Added support for `Size` and `Format` options and slightly improved format detection from canRead().
- Removed conversion to ARGB32 on load (improved performace with RGBA images).
- Added result checks on writing.

With this MR, all plugins have minimal support for options.

src/imageformats/rgb_p.h

@@ -9,7 +9,9 @@
 #define KIMG_RGB_P_H
 
 #include <QImageIOPlugin>
+#include <QScopedPointer>
 
+class SGIImagePrivate;
 class RGBHandler : public QImageIOHandler
 {
 public:
@@ -19,7 +21,13 @@ public:
     bool read(QImage *image) override;
     bool write(const QImage &image) override;
 
+    bool supportsOption(QImageIOHandler::ImageOption option) const override;
+    QVariant option(QImageIOHandler::ImageOption option) const override;
+
     static bool canRead(QIODevice *device);
+
+private:
+    QScopedPointer<SGIImagePrivate> d;
 };
 
 class RGBPlugin : public QImageIOPlugin