fontselect: coretext: get fallback font family name via FreeType

Since commit be0d1613f79a95073d18d96a60e1394abf9316a2,
get_fallback can return a name from Core Text that is different
from all names that match_fonts registers for the same font.
This causes font fallback to fail as find_fonts fails to
find any font having the name received from get_fallback.
(Moreover, libass will keep retrying fallback for other glyphs,
and the coretext match_fonts will keep readding the same fonts
over and over again.)

Commit 152d0484e98f118d01987138695cf40579c9e297 attempted
to fix font fallback by getting a full name from Core Text
instead of a family name when it was noticed that Core Text's
family name can come from TT_NAME_ID_TYPOGRAPHIC_FAMILY
in addition to TT_NAME_ID_FONT_FAMILY, which we fetch in
get_font_info, but the problem goes deeper: Core Text can return
Macintosh-platform names that don't match Microsoft-platform
names, or the font might have no Microsoft-platform names at all.
Furthermore, returning a full name breaks style matching:
get_fallback does not know about weight, slant etc. and is
expected to return a whole family of fonts that will be
lazy-loaded in its entirety (if applicable) and searched
through for the best stylistic match by find_fonts.

To fix fallback while preserving maximum name portability
in the spirit of be0d1613f79a95073d18d96a60e1394abf9316a2,
use the same API in get_fallback as is used in match_fonts
to look up a family name.

Note: this could be more efficient if ass_get_font_info
could be told to return just an arbitrary family name.

This fixes https://github.com/libass/libass/issues/457.

1 files changed, 2 insertions, 0 deletions

diff --git a/libass/ass_fontselect.h b/libass/ass_fontselect.h
index d029b0b..942ea00 100644
--- a/libass/ass_fontselect.h
+++ b/libass/ass_fontselect.h
@@ -144,12 +144,14 @@ typedef void    (*SubstituteFontFunc)(void *priv, const char *name,
  * fallbacks.
  *
  * \param priv font provider private data
+ * \param lib ASS_Library instance
  * \param family original font family name (try matching a similar font) (never NULL)
  * \param codepoint Unicode codepoint (UTF-32)
  * \return output font family, allocated with malloc(), must be freed
  *         by caller.
  */
 typedef char   *(*GetFallbackFunc)(void *priv,
+                                   ASS_Library *lib,
                                    const char *family,
                                    uint32_t codepoint);