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
The previous "the endpoints are inclusive" comment seems confusing. It's true
that the bounding box's max X equals the right-most coordinate, which suggests
<= instead of <, but that node's coordinate is itself exclusive. Consider the
solid 1-pixel square: (0, 0), (64, 0), (64, 64), (0, 64) in fixed.Point26_6
coordinates. The right-most coordinate is 64, and the bounding box's max X
equals 64, but rasterizing that square only affects sub-pixels up to but not
including 64.
Instead, it seems accurate to follow the fixed.Rectangle26_6 description, in
that the max values are exclusive.
