Each subpicture region is merged into one for libva 0.31.0 and applied
on top of the display surface by the GPU.
.
Higher versions of the libva library can handle multiple subpicture
region associations on a surface with vaAssociateSubpicture2() function.

- subtitles are readable
- some are clipped, this needs fixing
- crash in vlc_memcpy() (line: 545 of vaapi_x11.c file)
- fix positioning of subtitles (region->i_align)
- alpha is applied when available
- chroma is applied when available