[fontscan] Provide precise Location in FontMap.AddFace

[benoitkugler]

I use FontMap.AddFace to have control about the description registered with a font, but I would also control the file identifier.
That is, I would like to also pass a Location. For now, the package generate a synthetic file name with
out.Location.File = fmt.Sprintf("%v", md) (md being the font description provided by the caller).

Would you be OK to change the signature to AddFace(face font.Face, id Location, md meta.Description) ?

The previous behavior could be easily obtained with
AddFace(face font.Face, Location{File: fmt.Sprintf("%v", md)}, md meta.Description)

Pinging @whereswaldon as code author

[whereswaldon]

I have no objection; do it!