[Xwt.Wpf] Use a transparent background for spinners.
[Xwt.Wpf] Use a transparent background for spinners.

Previously they would be contained in a white rectangle thus not blending in their container.

residuum/xwt,TheBrainTech/xwt,steffenWi/xwt,mminns/xwt,iainx/xwt,cra0zy/xwt,mono/xwt,antmicro/xwt,lytico/xwt,akrisiun/xwt,hwthomas/xwt,mminns/xwt,directhex/xwt,sevoku/xwt,hamekoz/xwt