Notify data changes using Observer pattern

This pattern, in contrast with callbacks, allows us to clearly see
the dependency and hierarchy between the objects. It will also help
on writing unit tests as we can count how many times a notification
was triggered.

src/mbgl/map/map_context.cpp

@@ -168,10 +168,7 @@ void MapContext::updateTiles() {
     if (!style) return;
     for (const auto& source : style->sources) {
         source->update(data, transformState, *style, *glyphAtlas, *glyphStore, *spriteAtlas,
-                       getSprite(), *texturePool, [this]() {
-            assert(Environment::currentlyOn(ThreadType::Map));
-            triggerUpdate();
-        });
+                       getSprite(), *texturePool);
     }
 }