[Gtk] Fix a Cairo.LinearGradient leak
[Gtk] Fix a Cairo.LinearGradient leak

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