[Xwt] Fix Rectangle.Union wrt to empty surfaces.
[Xwt] Fix Rectangle.Union wrt to empty surfaces.

A union of an empty rectangle and a non-empty one should yield the empty one. Previously the method could inject some junk 0 values for x or y in that case.

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