Computes the width of the text in pixels in a CSS font as VexFlow sets it, e.g. "10pt Arial", whose family can also be a generic family or a list, e.g. "10pt Courier New, monospace". Optional, so that measurers implementing the interface without it keep working. Without it, the texts in EngravingRules.VexFlowTextFontFamily get as much space as in VexFlow's fonts.
Created by Matthias on 21.06.2016.