Features take a value (tag=N) in style commands, SVG and native files. Runs that
differ only in their feature lists are shaped in one buffer with ranged features, so
an alternate on one letter of an Arabic word keeps the word joined. FontFace reads
GSUB once (alternate sets, singles, the feature reaching a glyph), names unencoded
glyphs, and alternates_in_context finds a character's alternates in its word.
text.setRangeStyle takes glyphAlternate: null.