Initialize wxImageFileProperty correctly when file is given in the ctor.

Previously the file had to be specified later and the value passed to the ctor
was ignored.

Closes #15456.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74729 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/propgrid/advprops.h

@@ -329,6 +329,10 @@ public:
 protected:
     wxBitmap*   m_pBitmap; // final thumbnail area
     wxImage*    m_pImage; // intermediate thumbnail area
+
+private:
+    // Initialize m_pImage using the current file name.
+    void LoadImageFromFile();
 };
 
 #endif