[ios,macos,android,qt] use shared threadpool

platform/default/mbgl/util/shared_thread_pool.cpp

@@ -0,0 +1,14 @@
+#include "shared_thread_pool.hpp"
+
+namespace mbgl {
+
+std::shared_ptr<ThreadPool> sharedThreadPool() {
+    static std::weak_ptr<ThreadPool> weak;
+    auto pool = weak.lock();
+    if (!pool) {
+        weak = pool = std::make_shared<ThreadPool>(4);
+    }
+    return pool;
+}
+
+} // namespace mbgl