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

src/imageformats/raw.cpp

@@ -444,7 +444,9 @@ void setParams(QImageIOHandler *handler, LibRaw *rawProcessor)
     auto &&rawparams = rawProcessor->imgdata.rawparams;
 #endif
     // Select one raw image from input file (0 - first, ...)
-    rawparams.shot_select = handler->currentImageNumber();
+    if (handler->currentImageNumber() > -1) {
+        rawparams.shot_select = handler->currentImageNumber();
+    }
 
     // *** Set processing parameters
 
@@ -722,6 +724,7 @@ RAWHandler::RAWHandler()
     : m_imageNumber(0)
     , m_imageCount(0)
     , m_quality(-1)
+    , m_startPos(-1)
 {
 }
 
@@ -738,6 +741,15 @@ bool RAWHandler::read(QImage *image)
 {
     auto dev = device();
 
+    // set the image position after the first run.
+    if (!dev->isSequential()) {
+        if (m_startPos < 0) {
+            m_startPos = dev->pos();
+        } else {
+            dev->seek(m_startPos);
+        }
+    }
+
     // Check image file format.
     if (dev->atEnd()) {
         return false;
@@ -819,7 +831,7 @@ bool RAWHandler::jumpToNextImage()
 
 bool RAWHandler::jumpToImage(int imageNumber)
 {
-    if (imageNumber >= imageCount()) {
+    if (imageNumber < 0 || imageNumber >= imageCount()) {
         return false;
     }
     m_imageNumber = imageNumber;

src/imageformats/raw_p.h

@@ -74,6 +74,12 @@ private:
      * When the quality is -1, default quality is used.
      */
     qint32 m_quality;
+
+    /*!
+     * \brief m_startPos
+     * The initial device position to allow multi image load (cache value).
+     */
+    qint64 m_startPos;
 };
 
 class RAWPlugin : public QImageIOPlugin