Viewer: draw the rendered image directly instead of via a QPixmap

Every recolour ended with QPixmap::fromImage(), which allocates a second
full-size buffer and converts the whole image into the screen format. That
conversion was the largest single cost left in the colouring path.

Replace QGraphicsPixmapItem with a small item that paints qimg_buffer_ with
QPainter::drawImage. The buffer is already what the raster engine wants, so
nothing is converted or copied. The item declares its opaque area, as the
pixmap item did, so the view still skips the background fill underneath it,
and it turns SmoothPixmapTransform off before drawing to keep the
nearest-neighbour sampling QGraphicsPixmapItem gave us by default -- zoomed-in
detector pixels stay sharp squares.

GeneratePixmap is renamed RenderImage: it no longer makes a pixmap.

18.1 Mpx recolour: 22 -> 5.6 ms (28.0 ms before this series).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

viewer/image_viewer/JFJochAzIntImage.cpp

@@ -68,8 +68,8 @@ void JFJochAzIntImage::imageLoaded(std::shared_ptr<const JFJochReaderImage> in_i
         emit backgroundChanged(background);
         emit foregroundChanged(foreground);
 
-        // Generate pixmap and redraw using base class functionality
-        GeneratePixmap();
+        // Render the image and redraw using base class functionality
+        RenderImage();
         Redraw();
         CalcROI();
     } else {