[WPF] Fix CanvasBackend dirty rects exception
[WPF] Fix CanvasBackend dirty rects exception

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