cmake: revert to C++14

I've modified some of the code so our C++ version can decrease back to C++14. If this fixes the OTIO linker issue... well that sucks. Hopefully whatever the issue is gets resolved some time so that we can upgrade to C++17

app/node/generator/polygon/polygon.cpp

@@ -152,6 +152,18 @@ void PolygonGenerator::GenerateFrame(FramePtr frame, const GenerateJob &job) con
   }
 }
 
+template<typename T>
+NodeGizmo *PolygonGenerator::CreateAppropriateGizmo()
+{
+  return new T(this);
+}
+
+template<>
+NodeGizmo *PolygonGenerator::CreateAppropriateGizmo<PointGizmo>()
+{
+  return AddDraggableGizmo<PointGizmo>();
+}
+
 template<typename T>
 void PolygonGenerator::ValidateGizmoVectorSize(QVector<T*> &vec, int new_sz)
 {
@@ -168,11 +180,7 @@ void PolygonGenerator::ValidateGizmoVectorSize(QVector<T*> &vec, int new_sz)
 
     if (old_sz < new_sz) {
       for (int i=old_sz; i<new_sz; i++) {
-        if constexpr(std::is_same_v<T, PointGizmo>) {
-          vec[i] = AddDraggableGizmo<T>();
-        } else {
-          vec[i] = new T(this);
-        }
+        vec[i] = static_cast<T*>(CreateAppropriateGizmo<T>());
       }
     }
   }