refine release logics

Runtime/ui/renderer/cmdbufferCanvas/rendering/canvas_computebuffer_utils.cs

         }
 
         static void tryReleaseComputeBuffer() {
-            _instanceNum--;
-
             if (!supportComputeBuffer) {
                 return;
             }

Runtime/ui/renderer/cmdbufferCanvas/rendering/canvas_impl.cs

                 this._layers.Clear();
             }
 
+            _instanceNum--;
             tryReleaseComputeBuffer();
         }