Support linear gradient with multi-position color stops

[zsofitoth]

We should extend the parser to support this case:

background: linear-gradient(to right, white 30%, orange 60% 70%, blue 70%);

Visually, this looks like a gradual transition from white to orange between 30% and 60%, while between 0 and 30% the color is white, after 60% it's orange and after 70% it's blue.

See specification on color stop lists.