How to render letters on pieces?

[mephitrpg]

I edited /render.go setting showPieceLabels = true at line 14 then I launched go run cmd/render/main.go BCCPooBDDPooAAEPooooEFFQGGHoIQJJHoIQ out.png and tried other changes but it seems it doesn't apply my edits: I can't see letters.

Note: I don't know Go/Dart/C++/Python/Java, but I'm a developer and I know JavaScript and Node.

Thanks