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