Current NanoVG implementation doesn't support filled concave paths

NanoVG is used to draw many 2D shapes to the screen, including from the scripting graphics library.
While (as far as I can tell) there's currently nowhere in the code that tries to draw a concave path, this would be useful to let scripters draw arbitrary shapes to screen, which was a feature I was trying to add, but failed. If the paths are unfilled, they're drawn fine (ie, just the line strokes). If they are filled and convex, they're also drawn correctly. But if they're concave, nothing is drawn.

Upon some research, I found this:
https://github.com/memononen/nanovg/issues/461

According to the author, NanoVG needs to be drawn to a surface with a stencil buffer.

I tried looking into the code that initializes NanoVG and the drawing context, but I don't know enough to fix this.