Fix GerbView zoom-to-fit

gerbview/gerbview_frame.h

@@ -128,6 +128,12 @@ public:
     ///> @copydoc EDA_DRAW_FRAME::SetGridColor()
     virtual void SetGridColor( COLOR4D aColor ) override;
 
+    const BOX2I GetDocumentExtents() const override
+    {
+        wxASSERT( m_gerberLayout );
+        return m_gerberLayout->ViewBBox();
+    }
+
 protected:
     GERBER_LAYER_WIDGET*    m_LayersManager;