[core] current scheduler

- Adds a way to set the current scheduler on the thread to be used whenever a mailbox is created that needs to reply on this thread

cmake/core-files.cmake

@@ -8,6 +8,7 @@ set(MBGL_CORE_FILES
     include/mbgl/actor/message.hpp
     include/mbgl/actor/scheduler.hpp
     src/mbgl/actor/mailbox.cpp
+    src/mbgl/actor/scheduler.cpp
 
     # algorithm
     src/mbgl/algorithm/covered_by_children.hpp

include/mbgl/actor/scheduler.hpp

@@ -21,18 +21,21 @@ class Mailbox;
       Subject to these constraints, processing can happen on whatever thread in the
       pool is available.
 
-    * `RunLoop` is a `Scheduler` that is typically used to create a mailbox and
-      `ActorRef` for an object that lives on the main thread and is not itself wrapped
-      as an `Actor`:
-
-        auto mailbox = std::make_shared<Mailbox>(*util::RunLoop::Get());
+    * `Scheduler::GetCurrent()` is typically used to create a mailbox and `ActorRef`
+      for an object that lives on the main thread and is not itself wrapped an
+      `Actor`. The underlying implementation of this Scheduler should usually be
+      a `RunLoop`
+        auto mailbox = std::make_shared<Mailbox>(*Scheduler::Get());
         Actor<Worker> worker(threadPool, ActorRef<Foo>(*this, mailbox));
 */
-
 class Scheduler {
 public:
     virtual ~Scheduler() = default;
     virtual void schedule(std::weak_ptr<Mailbox>) = 0;
+
+    // Set/Get the current Scheduler for this thread
+    static Scheduler* GetCurrent();
+    static void SetCurrent(Scheduler*);
 };
 
 } // namespace mbgl

platform/default/thread_local.cpp

@@ -58,8 +58,8 @@ void ThreadLocal<T>::set(T* ptr) {
     }
 }
 
-template class ThreadLocal<RunLoop>;
 template class ThreadLocal<BackendScope>;
+template class ThreadLocal<Scheduler>;
 template class ThreadLocal<int>; // For unit tests
 
 } // namespace util

src/mbgl/actor/scheduler.cpp

@@ -0,0 +1,19 @@
+#include <mbgl/actor/scheduler.hpp>
+#include <mbgl/util/thread_local.hpp>
+
+namespace mbgl {
+    
+static auto& current() {
+    static util::ThreadLocal<Scheduler> scheduler;
+    return scheduler;
+};
+
+void Scheduler::SetCurrent(Scheduler* scheduler) {
+    current().set(scheduler);
+}
+
+Scheduler* Scheduler::GetCurrent() {
+    return current().get();
+}
+
+} //namespace mbgl