Looks like it gets triggered inside XftFontLoadGlyphs

   -> …/.libs/libXft.so.2.3.2(XftFontLoadGlyphs+0x716) [0x7f415551fd16]
   -> …/.libs/libXft.so.2.3.2(XftGlyphExtents+0x14b) [0x7f415551cf6b]
   -> [backtrace enters emacs proper]
   -> … (xftfont_text_extents)

but it would appear that the cause is that Xft can't process colour fonts
like Noto Color Emoji.

So we either have to:

   - hack on libXft until it can, and make that a requirement
   - use some other mechanism for colour fonts
   - exclude fonts like N-C-E /before/ we get to Xft so we don't crash

https://github.com/googlei18n/noto-emoji/issues/183