[core, ios, macos] Add image accessor to MGLStyle (#7096)

* [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.

src/mbgl/map/map.cpp

@@ -896,6 +896,13 @@ void Map::removeImage(const std::string& name) {
     impl->onUpdate(Update::Repaint);
 }
 
+const SpriteImage* Map::getImage(const std::string& name) {
+    if (impl->style) {
+        return impl->style->spriteAtlas->getSprite(name).get();
+    }
+    return nullptr;
+}
+
 #pragma mark - Defaults
 
 std::string Map::getStyleName() const {