[cairo] cairo_ft_font_face_create_for_ft_face Subtlety

[Lawrence D'Oliveiro]

The doc for cairo_ft_font_face_create_for_ft_face
<http://cairographics.org/manual/cairo-FreeType-Fonts.html#cairo-ft-font-face-create-for-ft-face>
says it “creates a new font face ... from a pre-opened FreeType face”.
What it doesn’t say is that, if you give it the same FT_Face twice, it
will return the same font_face_t.

You need to watch out for this, because the set_user_data dance will be
incorrect if you blindly do it every time. You need to call
get_user_data first to check if you have already attached a destroy
callback to this font_face_t, and skip doing it again if so.

I discovered this in the process of messing about with Multiple
Master/TrueType GX fonts. It looks like Cairo’s caching interferes
somewhat with the behaviour of these: to get around this, you have to
load the same font multiple times so that you can set different values
for the design coordinates on each copy.