Use correct visual attributes for wxSpinCtrl.

The default colours for this control are wxTextCtrl-like and not wxButton-like
(which is the default). Override GetDefaultAttributes() to indicate this and
to avoid returning a wrong background from wxControl::DoMSWControlColor() when
just a foreground is set.

See #1691.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61858 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/spinctrl.h

@@ -79,6 +79,18 @@ public:
     // wxSpinButton doesn't accept focus, but we do
     virtual bool AcceptsFocus() const { return wxWindow::AcceptsFocus(); }
 
+    // we're like wxTextCtrl and not (default) wxButton
+    virtual wxVisualAttributes GetDefaultAttributes() const
+    {
+        return GetClassDefaultAttributes(GetWindowVariant());
+    }
+
+    static wxVisualAttributes
+    GetClassDefaultAttributes(wxWindowVariant variant = wxWINDOW_VARIANT_NORMAL)
+    {
+        return GetCompositeControlsDefaultAttributes(variant);
+    }
+
     // for internal use only
 
     // get the subclassed window proc of the buddy text