Fix Design tab not painting on hardware GL after a page switch

request_repaint() only invalidates the canvas (Refresh()) on the hardware-GL
path and relies on a wxEVT_PAINT to follow. When the notebook re-shows the
Design page, that invalidation is issued mid-show and dropped: no paint event
arrives, the canvas never renders, and the pane stays blank until another tab
switch forces an expose. Software GL renders directly, so it never showed there.

force_repaint() defers past the show, then Refresh() + Update() for a
synchronous paint. Called from on_tab_shown().

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BVzKmX6Y1aEteit1HTXG4Q

src/slic3r/GUI/DesignCanvas.cpp

@@ -193,6 +193,20 @@ void DesignCanvas::request_repaint()
     }
 }
 
+void DesignCanvas::force_repaint()
+{
+    if (m_canvas == nullptr || m_canvas_widget == nullptr)
+        return;
+
+    CallAfter([this]() {
+        if (m_canvas == nullptr || m_canvas_widget == nullptr)
+            return;
+        m_canvas->set_as_dirty();
+        m_canvas_widget->Refresh();
+        m_canvas_widget->Update();   // synchronous: an expose may never come after a page show
+    });
+}
+
 void DesignCanvas::reload(bool keep_view)
 {
     m_canvas->reset_volumes();