GhPython mesh artist issues with color handling

The GH python mesh artist makes too many assumptions about color handling, specifically, there are (at least two issues):

* If a color is specified, it is converted to `rgb255` when creating the object, which discards the alpha value of the color. This could be fixed by changing all `rgb255` instances to `rgba255`
* It seems impossible to not define a color and get Rhino's default color. This might actually be intentional, but it's a bit of a questionable default behavior.