[WPF] Fix crash in desired size calculation
[WPF] Fix crash in desired size calculation

When exiting user code, don't run the queued actions if an exit callback
has been registered. This avoids running the actions too early. It fixes
a crash in WPF, caused by calling MeasureOverride in an unsupported sequence.

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