Remove implicit conversion of wxColour to GdkRGBA* in wxGTK

This was dangerous because it allowed the code accidentally
dereferencing wxColour to silently compile -- and do a completely wrong
thing.

Just add a function returning GdkRGBA explicitly and use it.

See #26751.

include/wx/gtk/colour.h

@@ -39,7 +39,7 @@ public:
 
     // Implementation part
 #ifdef __WXGTK3__
-    operator const GdkRGBA*() const;
+    const GdkRGBA* GTKGetRGBA() const;
 #else
     void CalcPixel( GdkColormap *cmap );
     int GetPixel() const;