Use a vlc_vector for SPU regions instead of vlc_list

On top of !4483 (merged)

Following the discussions on !4465 (merged), the container was kept to a vlc_list. Using a vlc_vector has some pro's and con's:

• it makes each subpicture_region_t 2 pointers smaller.
• it provides the size of the container right away
• it requires a check of allocation at every push (can be reserved early if the size is known)