Lazily load font face variations

[burg]

Currently, the FontList::refresh() method marches over all font faces installed on the system and creates CTFont, FontEntry, etc for it. If possible with CoreText APIs, we should instead eagerly create FontFamily entries, but lazily create the FontEntries whenever the FontFamily is queried.

This is approximately what Gecko does, but they use the ATS API instead of CoreText. The main challenge is getting reliable family names without creating CTFont instances, and then going from family name to CTFont instance using descriptors or whatever.

Promising API:

• CTFontManagerCopyAvailableFontFamilyNames

[burg]

Gecko does the same thing using Fontconfig:

• updating the list of font family names http://dxr.mozilla.org/mozilla-central/gfx/thebes/gfxFontconfigUtils.cpp.html#l561
• updating the faces in the family http://dxr.mozilla.org/mozilla-central/gfx/thebes/gfxFontconfigUtils.cpp.html#l320

[dhedlund]

This was addressed in 905ad4a when load_variations_for_family got fleshed out; all three platforms appear to have working implementations. A quick way to verify this is via the already embedded debug statements.

The following verifies FontEntries are being created lazily under Linux:

$ RUST_LOG=gfx ./servo -x ../src/test/html/about-mozilla.html 2>&1 | grep -o 'Creating new Font\(Family\|Entry\)' | uniq -c
    254 Creating new FontFamily
      4 Creating new FontEntry

[larsbergstrom]

Thanks for checking on this!