selfImFontGlyphRangesBuilder handle on which this method operates.
rangesUint16Array of inclusive [first, last] pairs followed by 0.
getGlyphRanges* helpers omit that terminator: append it explicitly with
new Uint16Array([...getGlyphRangesDefault(atlas), 0]).
The builder reads the ranges during this call; null is not accepted.
Add character ranges to the glyph-range builder.