function symbolNode(value, height, depth, italic, skew, classes, style) {
    this.value = value || "";
    this.height = height || 0;
    this.depth = depth || 0;
    this.italic = italic || 0;
    this.skew = skew || 0;
    this.classes = classes || [];
    this.style = style || {};
    this.maxFontSize = 0;
}
A symbol node contains information about a single symbol. It either renders
to a single text node, or a span with a single text node in it, depending on
whether it has CSS classes, styles, or needs italic correction.