assrender: fix smooth scaling by disabling hinting
When ass hinting value is set to anything other than NONE, subtitles cannot use smooth scaling, thus all animations will jitter.
The libass author warns about possibility of breaking some scripts when it is enabled, so lets do what is recommended and disable it to get the smooth scaling working.
Attached sample video to show the difference (English text is rendered).
Before: assrender-before
After: assrender-after
Edited by Rafał Dzięgiel