fill-opacity vs. fill

[rettinghaus]

Why do we have the opacity in Verovio's brush class? As far as I see we only use this to fill a shape or not, so basically this is a boolean. Are there any plans to really use opacity here? Otherwise we could reduce the class to only hold the color (which is currenColor in most cases) and set fill="none" for AxTRANSPARENT. This would simplify this a bit.