message: Add OSPSG_INTERFACE to Scheduler classes to fix Windows build
diff: #include <functional>
  #include <map>
  +#include "Node.h" // for OSPSG_INTERFACE
  +
  namespace ospray {
  namespace sg {
  namespace scheduler {
  @@ -30,7 +32,7 @@ using Function = std::function<void(SchedulerPtr)>;
  using FunctionPtr = std::shared_ptr<Function>;
  -class Scheduler : public std::enable_shared_from_this<Scheduler> {
  +class OSPSG_INTERFACE Scheduler : public std::enable_shared_from_this<Scheduler> {
  protected:
  // passkey idiom https://chromium.googlesource.com/chromium/src/+/HEAD/docs/patterns/passkey.md
  //
  @@ -84,7 +86,7 @@ private:
  };
  -class Instance : public std::enable_shared_from_this<Instance> {
  +class OSPSG_INTERFACE Instance : public std::enable_shared_from_this<Instance> {
  public:
  Instance(SchedulerPtr _scheduler, size_t _id, const std::string &_name)
  : scheduler(_scheduler)
  @@ -110,7 +112,7 @@ private:
  };
  -class Task : public std::enable_shared_from_this<Task> {
  +class OSPSG_INTERFACE Task : public std::enable_shared_from_this<Task> {
  public:
  Task(InstancePtr _instance, const std::string &_name, FunctionPtr _fcn)
  : instance(_instance)
  