[GTK] Fix bitmap conversion issue
[GTK] Fix bitmap conversion issue

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