build
fun buildGradientColor(zoom: Int, progress: Double, properties: Map<String, Any?> = emptyMap(), featureState: Map<String, Any?>? = null, geometryType: MvtGeometryType? = null): Color?
Sample lineGradient at the given progress (arc-length parameter ∈ 0, 1) for one feature. Returns null when the rule has no line-gradient.