Port shield library to iOS

[1ec5]

Some native iOS applications like Go Map!! display OSM Americana as a basemap using the generated style JSON file. Unfortunately, they have to omit the style’s signature features, such as route shields, because they require the maplibre-shield-generator library, which is only compatible with JavaScript applications.

To better support mobile users, we should port the shield library to iOS. This port would be written in Swift. It would read the same data as the JavaScript library but use Core Graphics instead of the Canvas API to perform drawing and compositing operations.