libvlc: use SUBPICTURE_ALIGN_xxx for "video-title-position"
We already use flags to fill some arrays like AOUT_MIX_MODE_xxx or VOUT_ALIGN_xxx.
The downside is that we cannot change the defines without breaking the libvlc API. But SUBPICTURE_ALIGN_xxx defines are also used by many module as source for parameters, so these would break as well.