libass: create a dummy dummy to tell the core the regions will be absolute

The core uses IsSubpicInVideo() to determine if the subpicture should be rendered inside the video. In most cases the subpictures with the b_subtitle flag (start and stop times) signal a subtitle SPU. But for libass it's not supposed to be rendered outside of the video. It has to match the video location.

IsSubpicInVideo() can't tell if the regions are absolute (positioned inside the video) unless there's an actual region. So we create a dummy one with the b_absolute flag.

A cleaner way would be to add a flag to subpicture_t to signal we allow subpictures in black bars or not. This will need to be checked for each subpicture source.