ifcviewer (GL minimal): --screenshot for parity diff with wgpu

Closes the other half of task #10. The wgpu minimal already wrote PNGs
via wgpuCommandEncoderCopyTextureToBuffer + mapAsync; the GL backend
now has the equivalent via glReadPixels on the back buffer just before
swapBuffers.

  - ViewportWindow::captureNextFrameToPng(path, quit_after=true) queues
    a one-shot capture. render() reads the default framebuffer at full
    pixel size (width * devicePixelRatio), flips bottom-up → top-down
    into a QImage::Format_RGBA8888, saves PNG, and optionally
    QCoreApplication::quit. Synchronous glReadPixels is fine here —
    pick is interactive and rare; not used per-frame.

  - ifcviewer-minimal --screenshot PATH wires through MinimalWindow
    just like --camera / --benchmark. Honoured after all loads complete
    (applyPendingBenchmark also drains pending_screenshot_).

Lets a parity script do:
    IfcViewerMinimal      foo.ifc      --camera A,B,C,D,E,F --screenshot gl.png
    IfcViewerWgpuMinimal  foo.ifcview  --camera A,B,C,D,E,F --screenshot wgpu.png
    # then pixel-diff with whatever (ImageMagick, PIL, etc.)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ifcviewer-minimal/MinimalWindow.cpp

@@ -127,8 +127,13 @@ void MinimalWindow::setPendingBenchmark(int frames) {
     pending_benchmark_ = frames;
 }
 
+void MinimalWindow::setPendingScreenshot(const QString& path) {
+    pending_screenshot_ = path;
+}
+
 void MinimalWindow::applyPendingBenchmark() {
-    if (pending_camera_.isEmpty() && pending_benchmark_ <= 0) return;
+    if (pending_camera_.isEmpty() && pending_benchmark_ <= 0
+        && pending_screenshot_.isEmpty()) return;
 
     if (!pending_camera_.isEmpty()) {
         QStringList parts = pending_camera_.split(',');
@@ -148,4 +153,9 @@ void MinimalWindow::applyPendingBenchmark() {
         viewport_->setBenchmarkFrames(pending_benchmark_);
         pending_benchmark_ = 0;
     }
+
+    if (!pending_screenshot_.isEmpty()) {
+        viewport_->captureNextFrameToPng(pending_screenshot_, /*quit_after=*/true);
+        pending_screenshot_.clear();
+    }
 }

src/ifcviewer-minimal/MinimalWindow.h

@@ -35,6 +35,10 @@ public:
     void addFiles(const QStringList& paths);
     void setPendingCamera(const QString& params);
     void setPendingBenchmark(int frames);
+    // One-shot framebuffer capture queued for the next render. Forwarded
+    // to ViewportWindow::captureNextFrameToPng; the viewport handles the
+    // glReadPixels + PNG save + optional QCoreApplication::quit.
+    void setPendingScreenshot(const QString& path);
 
 private slots:
     void onLoadStarted(uint32_t mid, QString display_name);
@@ -55,6 +59,7 @@ private:
 
     QString pending_camera_;
     int     pending_benchmark_ = 0;
+    QString pending_screenshot_;
 };
 
 #endif // MINIMALWINDOW_H

src/ifcviewer-minimal/main.cpp

@@ -44,6 +44,8 @@ int main(int argc, char* argv[]) {
         "Set camera: tx,ty,tz,dist,yaw,pitch", "params"});
     parser.addOption({{"b", "benchmark"},
         "Run N frames then print stats and exit", "frames"});
+    parser.addOption({{"s", "screenshot"},
+        "Render one frame after load, save to PATH as PNG, exit", "path"});
     parser.process(app);
 
     MinimalWindow window;
@@ -60,6 +62,9 @@ int main(int argc, char* argv[]) {
     if (parser.isSet("benchmark")) {
         window.setPendingBenchmark(parser.value("benchmark").toInt());
     }
+    if (parser.isSet("screenshot")) {
+        window.setPendingScreenshot(parser.value("screenshot"));
+    }
 
     return app.exec();
 }