Due to a quirk in the ASS format some tags do depend on the exact
storage size of the video not just the pixel aspect ratio, so tell
libass via ass_set_storage_size to achieve correct rendering.

For ASS, storage size refers to the size the video is encoded in (stored
at) with codec-level crop applied; further transforms like e.g.
anamorphic de-squeezing or container-level crop must not be applied.
This matches the used fmt_src values despite them being named
visible_{width,height} and the existing PAR calculation already relies
on this.

Fixes #26634

(cherry picked from commit 2608eb5c)