Log Message:
libass: updated to 0.17.3

libass (0.17.3)
* Fix 0.17.2 regression in the fontconfig fontprovider
  leading to undesirable widths being chosen from large typographic families
* Fix configure generated with slibtool-provided autoconf macros
* Fix "make check" for shared-only builds
* Constify some API parameters in a backwards-compatible manner
* Add new ass_malloc and ass_free API functions
* Tweak default optimization flags
* Speed up parsing of events with very long override blocks
* Improve handling of HarfBuzz-related failures