WinGui: default to burn-in for VobSub too (when the container is MP4).

This is for compatibility reason (VobSub in MP4 is supported by very few players or devices) and matches the MacGUI behavior
.


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5377 b64f7644-9d1e-0410-96f1-a4d463321fa5