Expose more options for downloading offline packs in macosapp

[1ec5]

macosapp can be used to test offline map functionality or generate offline packs to sideload onto iOS. Currently, it only supports downloading a tile pyramid from z0 up to the current zoom level. With #11447, the iOS and macOS map SDKs are gaining the ability to download packs that cover non-rectangular regions. macosapp should expose a way to download an offline pack whose region is an MGLShapeOfflineRegion.

The most natural UI for selecting a download region would be a drawing UI, which could be ported from a very small amount of old Swift 2 code for iOS in MBDrawingView.swift. Alternatively, macosapp could allow the user to open a GeoJSON file (#5483) defining the offline region.

/cc @asheemmamoowala @julianrex

[stale]

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