raw: fix multi image load

Fixes not loading a second image in the file. This patch allow code like the following.

QImageReader r(file);
do {
    auto qi = r.read();
    if (!qi.isNull()) {
        qi.save(QString("/tmp/%1_%2.tif")
                .arg(QFileInfo(file).baseName())
                .arg(r.currentImageNumber()));
    }
}
while (r.jumpToNextImage());

m_startPos is used to reposition the device if you decide to do a subsequent read: libraw wants it to be at the beginning of the RAW stream