Explicit Cell Editor Fix

Fixed a problem when trying to deactivate explicit cells through the editor since it doesn't allow you remove all cells and explicit cells can't be turned off if any cells haven't been removed. Sort of a chicken and egg problem.

engine/source/2d/assets/ImageAsset.cc

@@ -1686,7 +1686,7 @@ void ImageAsset::onTamlCustomWrite( TamlCustomNodes& customNodes )
     // Call parent.
     Parent::onTamlCustomWrite( customNodes );
 
-    if (mExplicitFrames.size() > 0)
+    if (mExplicitMode && mExplicitFrames.size() > 0)
     {
         // Add cell custom node.
         TamlCustomNode* pCustomCellNodes = customNodes.addNode( cellCustomNodeCellsName );