package/libass: bump version to 0.13.0

Quote from https://github.com/libass/libass/blob/master/Changelog


"* Drop ENCA support"

Therefore we remove the dependency to libenca.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>