libass recently switched the default from 1 to 0 for compatibility
with ASS scripts that rely on the historical/VSFilter default of 0.
libass does attempt to detect and avoid breaking scripts that rely
on the historic libass-only default of 1, but it doesn't cover tracks
created directly through the API, so set the header explicitly.
Fixes https://github.com/mpv-player/mpv/issues/7900.