Add glyphs to the named curve-set, creating it if it doesn't exist yet. Glyph ids
already present in the curve-set are skipped (the existing outline + atlas slot wins).
Safe to call between frames: the atlas grows in place and the next render uploads the
new glyphs.
Add glyphs to the named curve-set, creating it if it doesn't exist yet. Glyph ids already present in the curve-set are skipped (the existing outline + atlas slot wins). Safe to call between frames: the atlas grows in place and the next render uploads the new glyphs.