Investigate route gradient stop property declaration in NavigationMapView

[captainbarbosa]

Following up on #2377 (comment),

NavigationMapView declares two gradient stop dictionaries in the form of a tuple, used to calculate the traffic line gradients for both the main route line and its casing:

typealias RouteGradientStops = (line: [CGFloat: UIColor], casing: [CGFloat: UIColor])
private var routeGradientStops = RouteGradientStops(line: [:], casing: [:])

It was suggested in #2377 (comment) that this be changed to two parallel properties instead, such as:

private var routeGradientStops: [CGFloat: UIColor] = [:]
private var routeGradientCasingStops: [CGFloat: UIColor] = [:]

However, this declaration causes the gradient stops to be miscalculated - we should look into why this is happening.

Tuple	Two parallel properties

* note - the line caps are not rounded in these two screenshots, but since those were taken that has been corrected.

cc @1ec5