Revert "Revert "split renderSync and transition nudging to allow client view syncing""

This reverts commit 263f9fad30.

include/mbgl/map/map.hpp

@@ -67,7 +67,10 @@ public:
     void renderStill(StillImageCallback callback);
 
     // Triggers a synchronous or asynchronous render.
-    void renderSync();
+    bool renderSync();
+
+    // Nudges transitions one step, possibly notifying of the need for a rerender.
+    void nudgeTransitions(bool forceRerender);
 
     // Notifies the Map thread that the state has changed and an update might be necessary.
     void update(Update update = Update::Nothing);