directwrite: fix font collections

DirectWrite's FontFileStream does not actually use the data of a specific
font in a collection, which was an expectation of the existing code. It
simply returns a stream to the underlying file, collection or not. So we
need to get the index of the font. This needs to be done lazily as this
information is only available in a FontFace, which is expensive to
initialize.

Add a new optional font provider function for lazy initialization of the
index and use it. This is similar to the check_postscript callback.

Fixes libass#275.

v2: fix type of returned value.

1 files changed, 11 insertions, 0 deletions

diff --git a/libass/ass_fontselect.h b/libass/ass_fontselect.h
index 73d5265..8d6e779 100644
--- a/libass/ass_fontselect.h
+++ b/libass/ass_fontselect.h
@@ -71,6 +71,16 @@ typedef bool    (*CheckPostscriptFunc)(void *font_priv);
 typedef bool    (*CheckGlyphFunc)(void *font_priv, uint32_t codepoint);
 
 /**
+* Get index of a font in context of a font collection.
+* This function is optional and may be needed to initialize the font index
+* lazily.
+*
+* \param font_priv font private data
+* \return font index inside the collection, or 0 in case of a single font
+*/
+typedef unsigned    (*GetFontIndex)(void *font_priv);
+
+/**
  * Destroy a font's private data.
  *
  *  \param font_priv font private data
@@ -150,6 +160,7 @@ typedef struct font_provider_funcs {
     MatchFontsFunc      match_fonts;            /* optional */
     SubstituteFontFunc  get_substitutions;      /* optional */
     GetFallbackFunc     get_fallback;           /* optional */
+    GetFontIndex        get_font_index;         /* optional */
 } ASS_FontProviderFuncs;
 
 /*