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.hpp

@@ -245,6 +245,11 @@ public:
     // scheduled Refresh() is frequently dropped there. Backend cached on first use.
     // Public: DesignPanel calls it after a tree edit to force a frame on software GL.
     void request_repaint();
+    // Repaint synchronously, once the pending show/resize has settled. Needed when the
+    // notebook re-shows the Design page: an invalidation issued while the page is still
+    // being shown is dropped on hardware GL and no wxEVT_PAINT ever follows, leaving the
+    // canvas blank until another tab switch forces an expose.
+    void force_repaint();
 
 private:
     void reload(bool keep_view);