refine release logics

Author: zhuxingwei

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

@@ -31,8 +31,6 @@ static bool supportComputeBuffer {
         }
 
         static void tryReleaseComputeBuffer() {
-            _instanceNum--;
-
             if (!supportComputeBuffer) {
                 return;
             }

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

@@ -43,6 +43,7 @@ public void dispose() {
                 this._layers.Clear();
             }
 
+            _instanceNum--;
             tryReleaseComputeBuffer();
         }