[Mac] ProgressBar: fix Indeterminate mode
[Mac] ProgressBar: fix Indeterminate mode

Apparently StartAnimation() only has effect if Indeterminate
mode is enabled.

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