uxtheme: ExtCreateRegion does not work

In the implementation of GetThemeBackgroundRegion in uxtheme, we use ExtCreateRegion to create the region object that is returned however it doesn't work and we hack it by using many calls to CreateRectRgn and CombineRgn. Both versions are commited in trunk and someone can switch to using ExtCreateRegion by defining EXTCREATEREGION_WORKS. Also note that both versions are tested and work fine in windows