Fix creating wxButton using stock id with GTK

Broken by 85d63c3150 (Add a class derived from GtkImage to support HiDPI bitmaps, 2020-09-15)
See #19288

include/wx/gtk/private/image.h

@@ -31,3 +31,4 @@ public:
 };
 
 #define WX_GTK_IMAGE(obj) G_TYPE_CHECK_INSTANCE_CAST(obj, wxGtkImage::Type(), wxGtkImage)
+#define WX_GTK_IS_IMAGE(obj) G_TYPE_CHECK_INSTANCE_TYPE(obj, wxGtkImage::Type())

src/gtk/anybutton.cpp

@@ -192,7 +192,10 @@ void wxAnyButton::GTKDoShowBitmap(const wxBitmap& bitmap)
 
     wxCHECK_RET(GTK_IS_IMAGE(image), "must have image widget");
 
-    WX_GTK_IMAGE(image)->Set(bitmap);
+    if (WX_GTK_IS_IMAGE(image))
+        WX_GTK_IMAGE(image)->Set(bitmap);
+    else
+        gtk_image_set_from_pixbuf(GTK_IMAGE(image), bitmap.GetPixbuf());
 }
 
 wxBitmap wxAnyButton::DoGetBitmap(State which) const