vout_subpicture: fix early disappear issue in libaribcaption

This reverts changes in 7e659707 which caused all captions rendered by libaribcaption to disappear earlier than expected.

Correct timing after reverting the commit:

correct_timing

Wrong timing before reverting the commit:

wrong_timing

I doubt if reverting that commit is the right solution. I hope someone can come up with a better one after creating this MR.