For example, "Apple Color Emoji.ttc" has such a cmap. After this commit,
we can render its glyph for "U+0001F1E6 REGIONAL INDICATOR SYMBOL LETTER
A", which is clearly outside the Unicode BMP.

Note that this only lets us render black-and-white glyphs for out-of-BMP
runes. Rendering color glyphs (i.e. emoji) remains TODO.

Updates #45