package/libass: security bump to version 0.15

package/libass: security bump to version 0.15

- harfbuzz is mandatory since
  https://github.com/libass/libass/commit/f3e2c97e1818598afb0b1c7010003ffe4823ff21
- Fix CVE-2020-26682 (In libass 0.14.0, the `ass_outline_construct`'s
  call to `outline_stroke` causes a signed integer overflow.) through
  https://github.com/libass/libass/commit/676f9dc5b52ef406c5527bdadbcb947f11392929
  which does not apply cleanly over version 0.14.
  It should be noted that version 0.15 also fixes other integer
  overflows (which have no CVE assigned)
- Update indentation in hash file (two spaces)

https://github.com/libass/libass/releases/tag/0.15.0

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>