Fills a path with a colour.
void FillPath(PathView path, FillRule rule, in DisplayColor color)
The path in user space, exactly as constructed (see PathView): the backend closes open subpaths for the fill and disregards a trailing single-point subpath (ISO 32000-2 §8.5.3.3.1).
The rule that decides the inside: non-zero winding (§8.5.3.3.2) or even-odd (§8.5.3.3.3).
The nonstroking colour, unconverted.
ISO 32000-2 §8.5.3.1 and §8.5.3.3 (f, F, f*, and the fill of B, B*, b, b*).