Color is rendered incorrectly if it is passed via `datum`.

[lassepe]

On the current master, the color is rendered incorrectly if it is set via the datum property.

The bug can be reproduced using the following code:

using VegaLite: @vlplot
@vlplot() + @vlplot(mark = :point, x = rand(10), y = rand(10), color = {datum = "class 1"}) + @vlplot(mark = :point, x = rand(10), y = rand(10), color = {datum = "class 2"})

In Firefox and Chromium, this renders correctly as:

When using ElectronDisplay with the current master (e421241) the color channel is ignored:

Edit: This used to work with v0.8.