wgpu backend: --screenshot capability for visual verification

Pulls the capture half of task #10 forward so we stop flying blind from
stage 3 onward. WgpuViewportWindow gains captureNextFrameToPng(path);
the minimal driver wires it to a --screenshot PATH flag that renders
one frame, copies the surface texture back to host memory, writes a
PNG via QImage, and quits.

CopySrc is added to the surface configuration usage so the surface
texture can be the copy source. The texel-to-buffer copy honours
WebGPU's 256-byte bytes-per-row alignment by padding rows and stripping
the padding when assembling the QImage. Surface format 28 (BGRA8Unorm)
is byte-swapped to RGBA on the way into QImage::Format_RGBA8888;
RGBA8 surface formats are memcpy'd straight through.

Verified end-to-end on /tmp/basic.ifcview: 3 cube meshes/instances
render with depth, back-face cull, and the hemisphere-ambient + key+fill
lighting model — top face reads sky (bright), front faces read mid-tone,
exactly as the WGSL shading intended. The pixel-diff half of task #10
(comparing against a GL baseline) lands later when the GL minimal binary
gets an equivalent flag.

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

src/ifcviewer-wgpu/WgpuViewportWindow.h

@@ -75,6 +75,13 @@ public:
     // can re-invoke to re-frame.
     void viewAll();
 
+    // Queue a one-shot framebuffer capture: the next rendered frame is
+    // copied back to host memory and saved to `path` as PNG. If
+    // `quit_after` is true, QCoreApplication::quit() is called once the
+    // PNG is written. Use this for headless verification and pixel-diff
+    // parity testing against the GL backend.
+    void captureNextFrameToPng(const QString& path, bool quit_after = true);
+
 protected:
     void exposeEvent(QExposeEvent* event) override;
     void resizeEvent(QResizeEvent* event) override;
@@ -147,6 +154,10 @@ private:
     // subsequent loads from snapping the camera away from where the
     // user pointed it.
     bool initial_view_applied_ = false;
+
+    // Pending one-shot screenshot, captured at the end of the next render().
+    QString pending_screenshot_path_;
+    bool    pending_screenshot_quit_ = false;
 };
 
 #endif // WGPUVIEWPORTWINDOW_H