Viewer: do not render the magnifier close-up while it is closed

centerAt() checked isVisible(), but imageLoaded() did not, so every frame built
a SimpleImage over the whole detector image and ran it through the full
JFJochSimpleImage path -- convert to float, colour every pixel, redraw -- to
feed a 320x320 window that is closed by default and stays closed most of the
time.

Remember the frame instead and do the work in showEvent(). Holding the
shared_ptr also keeps alive the buffer that the SimpleImage's CompressedImage
points into, which it did not own.

Stepping 30 frames with the magnifier closed: 5545 -> 4770 ms CPU (-14%), on a
2.8 Mpx detector; the saving is per-pixel, so it grows with detector size. With
the magnifier open the cost is unchanged (5500 ms), which is what was being
paid unconditionally before.

Verified in the GUI: opening the magnifier still populates it, and it still
refreshes when the frame changes.

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

viewer/windows/JFJochMagnifierWindow.cpp

@@ -5,6 +5,7 @@
 #include "../image_viewer/JFJochSimpleImage.h"
 #include "../SimpleImage.h"
 
+#include <QShowEvent>
 #include <QTransform>
 
 JFJochMagnifierWindow::JFJochMagnifierWindow(QWidget *parent)
@@ -17,6 +18,26 @@ JFJochMagnifierWindow::JFJochMagnifierWindow(QWidget *parent)
 }
 
 void JFJochMagnifierWindow::imageLoaded(std::shared_ptr<const JFJochReaderImage> image) {
+    m_pending_image = std::move(image);
+    // The window is closed most of the time, and rendering a close-up nobody is looking at costs
+    // a full conversion and recolour of the whole detector image on every frame.
+    if (!isVisible()) {
+        m_pending_dirty = true;
+        return;
+    }
+    ApplyPendingImage();
+}
+
+void JFJochMagnifierWindow::showEvent(QShowEvent *event) {
+    JFJochHelperWindow::showEvent(event);
+    if (m_pending_dirty)
+        ApplyPendingImage();
+}
+
+void JFJochMagnifierWindow::ApplyPendingImage() {
+    m_pending_dirty = false;
+    const std::shared_ptr<const JFJochReaderImage> &image = m_pending_image;
+
     if (!image) {
         m_have_image = false;
         m_image->setImage(nullptr);

viewer/windows/JFJochMagnifierWindow.h

@@ -18,6 +18,15 @@ class JFJochMagnifierWindow : public JFJochHelperWindow {
     double m_magnification = 12.0;
     bool m_have_image = false;
 
+    // Building the close-up converts and colours the whole detector image, so it is only done
+    // while the window is actually up. The frame is remembered either way; holding the
+    // shared_ptr also keeps alive the buffer the SimpleImage points into.
+    std::shared_ptr<const JFJochReaderImage> m_pending_image;
+    bool m_pending_dirty = false;
+    void ApplyPendingImage();
+
+    void showEvent(QShowEvent *event) override;
+
 public:
     explicit JFJochMagnifierWindow(QWidget *parent = nullptr);