[cairo] Reference counting of cairo free type face

[Julian Viereck]

Hi there,

I try to add font support to the node-canvas [1] library, which uses 
cairo for rendering. I use the freetype2 library to load a font face and 
then use it in cairo. The font rendering is working, but I got stuck on 
memory management of the cairo font face. In particular, the cairo font 
face's reference counter is not reset to 1 at the point I expect it to be.

Here's what I'm doing. I concentrate on the essential parts only. Let 
"ref(cr_face)" be "cairo_font_face_get_reference_count(cr_face)" to get 
the internal reference counter for the cairo font face.

     // Create a free type font face from a font file.
     FT_New_Face(library, *filePath, faceIdx, &ft_face);
     // Create a cairo font face.
     cr_face = cairo_ft_font_face_create_for_ft_face(ft_face, 0);

     // At this point ref(cr_face) == 1.

     // Set the cairo font face on a cairo rendering context.
     cairo_set_font_face(ctx, cr_face);

     // At this point ref(cr_face) == 2. I guess that's okay, as the 
font is requird for rendering.

     cairo_text_path(ctx, str);

     // At this point ref(cr_face) == 4.

     cairo_destroy(ctx);

     // At this point ref(cr_face) == 3.

Shouldn't the reference count after the "cairo_destory" be 1 again and 
the cairo font face deleted? Do I need to perform some manually cleanup 
first?

Hope someone can help me out here and very best,

Julian

PS: I use cairo 1.12.8 and the freetype 2.4.10 library.

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[1]: https://github.com/LearnBoost/node-canvas