Clear background if background brush is invalid in wxGTK2 too

We only should avoid clearing the background if the brush is
transparent, not if it's not set.

This refines 6549d4c3c5

See #10273.

src/gtk/dcclient.cpp

@@ -1508,7 +1508,7 @@ void wxWindowDCImpl::Clear()
 
     if (!m_gdkwindow) return;
 
-    if (!m_backgroundBrush.IsOk() || m_backgroundBrush.GetStyle() == wxBRUSHSTYLE_TRANSPARENT)
+    if (m_backgroundBrush.IsTransparent())
         return;
 
     int width,height;