Add method for calculating number of tiles in tile pyramid

[1ec5]

MGLTilePyramidOfflineRegion should have a countOfTiles method that returns the number of tiles included in the tile pyramid. The implementation would rely on the mbgl::util::tileCount() function added in #9906.

This is the iOS/macOS equivalent to #11108 for Android. It would serve as a built-in alternative to mapbox/turf-swift#47 for applications already using the map SDK, including applications written in Objective-C.

/cc @captainbarbosa @kalimar @asheemmamoowala

[1ec5]

#11447 may end up generalizing MGLTilePyramidOfflineRegion so that it can cover a non-polygonal region of the map. #11267 tracks similarly generalizing mbgl::util::tileCount() to accept an arbitrary geometry, so it still makes sense to add a method to whatever becomes of MGLTilePyramidOfflineRegion.

[stale]

This issue has been automatically detected as stale because it has not had recent activity and will be archived. Thank you for your contributions.

[stale]

This issue has been automatically detected as stale because it has not had recent activity and will be archived. Thank you for your contributions.