osd_libass: set ScaledBorderAndShadow

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.

sub/osd_libass.c

@@ -132,6 +132,7 @@ static void create_ass_track(struct osd_state *osd, struct osd_object *obj,
     track->Timer = 100.;
     track->WrapStyle = 1; // end-of-line wrapping instead of smart wrapping
     track->Kerning = true;
+    track->ScaledBorderAndShadow = true;
 
     update_playres(ass, &obj->vo_res);
 }