Make Bucket::uploaded an atomic bool

This class is called from multiple threads, so we better make this
synchronization flag atomic.

src/mbgl/renderer/bucket.hpp

@@ -5,6 +5,8 @@
 #include <mbgl/util/noncopyable.hpp>
 #include <mbgl/util/mat4.hpp>
 
+#include <atomic>
+
 #define BUFFER_OFFSET(i) ((char*)nullptr + (i))
 
 namespace mbgl {
@@ -15,6 +17,8 @@ class TileID;
 
 class Bucket : private util::noncopyable {
 public:
+    Bucket() : uploaded(false) {}
+    
     // As long as this bucket has a Prepare render pass, this function is getting called. Typically,
     // this only happens once when the bucket is being rendered for the first time.
     virtual void upload() = 0;
@@ -33,7 +37,7 @@ public:
     virtual void swapRenderData() {}
 
 protected:
-    bool uploaded = false;
+    std::atomic<bool> uploaded;
 
 };