* [core] Add interface to get image from sprite atlas
* [tests] Add tests for Map::getImage
* [ios, macos] WIP: get MGLImage for name from style
* [ios, macos] Fixed -imageForName:
Convert from sprite images to platform images using the existing encodePNG() function, which is also used for printing. Allow -imageForName: to return nil without an assertion failure. Added a basic test.