Gradient fills

[edenh]

Would be nice to have linear gradient fills for fill and line types.

cc @samanpwbb

[samanpwbb]

This would be useful for a lot of things, but my impractical dream is to be able to do this:

Now for calculating gradient direction.... if you had a oneway value in your data you could do it :P

[wboykinm]

Not unprecedented, but it looks pretty damn hard.

[incanus]

This would be great for the oft-talked-about use case of running maps showing varying values in the path. It separates the library from the normal capabilities of just splitting a path into different-colored segments with hard color boundary edges.

[jfirebaugh]

Actually drawing the gradient wouldn't be that hard. The hard part is that for the motorway link use case you want topology-dependent style properties: "gradient between the colors of the two lines to which this line connects."

[lucaswoj]

Closed in favor of mapbox/mapbox-gl-style-spec#349