Function parseRgbObject

Tries to parse an object in forms: {r,g,b}, {red,green,blue}. Uses 'opacity', 'space' and 'unit' fields where available.

If 'units' is not specified, it tries to guess if it's relative (0..1) or 8-bit (0..255).

Normalises to an Rgb structure if it can, or returns an error.