_2p(index) {
    const [ x0,  y0 ] = this._origins[index];
    const x2p = x => x0 + x/this.upm*this.fontSize;
    const y2p = y => y0 + y/this.upm*this.fontSize;
    return [ x2p, y2p ];
  }
Glyph coordinates to gridCanvas project coordinates
@param { number } index Index of the glyph
@returns { [ (x: number) => number, (y: number) => number ] } 
x, y transformations