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, 14 insertions, 0 deletions

diff --git a/libass/ass_directwrite.c b/libass/ass_directwrite.c
index 45d2a2e..d15cbcd 100644
--- a/libass/ass_directwrite.c
+++ b/libass/ass_directwrite.c
@@ -352,6 +352,19 @@ static bool check_postscript(void *data)
 }
 
 /*
+ * Lazily return index of font. It requires the FontFace to be present, which is expensive to initialize.
+ */
+static unsigned get_font_index(void *data)
+{
+    FontPrivate *priv = (FontPrivate *)data;
+
+    if (!init_font_private_face(priv))
+        return 0;
+
+    return IDWriteFontFace_GetIndex(priv->face);
+}
+
+/*
  * Check if the passed font has a specific unicode character.
  */
 static bool check_glyph(void *data, uint32_t code)
@@ -719,6 +732,7 @@ static ASS_FontProviderFuncs directwrite_callbacks = {
     .destroy_provider   = destroy_provider,
     .get_substitutions  = get_substitutions,
     .get_fallback       = get_fallback,
+    .get_font_index     = get_font_index,
 };
 
 typedef HRESULT (WINAPI *DWriteCreateFactoryFn)(