Serialize GPU context disposal

PerfectNumbers.Core/Gpu/GpuContextPool.cs

@@ -115,10 +115,13 @@ public static class GpuContextPool
 		}
 
                 else
+                {
+                        lock (CreationLock)
                         {
                                 ctx.Dispose();
                         }
                 }
+        }
 
     public struct GpuContextLease : IDisposable
     {