Add conditional showing of Clear Debug Draw buttons only if there is anything to clear

#2989

Source/Engine/Debug/DebugDraw.cpp

@@ -347,6 +347,11 @@ struct DebugDrawContext
     DebugDrawData DebugDrawDepthTest;
     Float3 LastViewPos = Float3::Zero;
     Matrix LastViewProj = Matrix::Identity;
+
+    inline int32 Count() const
+    {
+        return DebugDrawDefault.Count() + DebugDrawDepthTest.Count();
+    }
 };
 
 namespace
@@ -739,6 +744,13 @@ void DebugDraw::SetContext(void* context)
     Context = context ? (DebugDrawContext*)context : &GlobalContext;
 }
 
+bool DebugDraw::CanClear(void* context)
+{
+    if (!context)
+        context = &GlobalContext;
+    return ((DebugDrawContext*)context)->Count() != 0;
+}
+
 #endif
 
 Vector3 DebugDraw::GetViewPos()