[core] Followup fix to #3037

The previous fix resulted in annotations sometimes never being added.
This would happen if, during the execution of MapContext::update where 
Update::Annotations was set, the style itself was loaded but some of
its subresources (sprite or sources) were not.

#3082 is a follow up issue to make MapContext::update less error-prone.

src/mbgl/map/map_context.cpp

@@ -169,7 +169,7 @@ void MapContext::update() {
 
     data.setAnimationTime(Clock::now());
 
-    if (style->isLoaded() && updateFlags & Update::Annotations) {
+    if (style->loaded && updateFlags & Update::Annotations) {
         data.getAnnotationManager()->updateStyle(*style);
         updateFlags |= Update::Classes;
     }

src/mbgl/style/style.hpp

@@ -100,7 +100,6 @@ private:
     void emitTileDataChanged();
     void emitResourceLoadingFailed(std::exception_ptr error);
 
-    bool loaded = false;
     bool shouldReparsePartialTiles = false;
 
     Observer* observer = nullptr;
@@ -112,6 +111,7 @@ private:
     bool hasPendingTransitions = false;
 
 public:
+    bool loaded = false;
     Worker workers;
 };