Fix an issue with the background in wxGenericAnimationCtrl

Static bitmap with alpha channel must also be drawn on the memory DC
instead of being used directly, just as the bitmaps with a mask.

src/generic/animateg.cpp

@@ -454,8 +454,9 @@ void wxGenericAnimationCtrl::DisplayStaticImage()
     if (m_bmpStaticReal.IsOk())
     {
         // copy the inactive bitmap in the backing store
-        // eventually using the mask if the static bitmap has one
-        if ( m_bmpStaticReal.GetMask() )
+        // eventually using the mask or the alpha if the static
+        // bitmap has one
+        if ( m_bmpStaticReal.GetMask() || m_bmpStaticReal.HasAlpha() )
         {
             wxMemoryDC temp;
             temp.SelectObject(m_backingStore);