Regenerate roughjs shape only when the item is updated (#316)

* Regenerate roughjs shape only when the item is updated

* Remove shape object during export and history undo/redo

* Remove shape element during copying

* Fix shape generation during creation

src/history.ts

@@ -7,7 +7,10 @@ class SceneHistory {
 
   generateCurrentEntry(elements: readonly ExcalidrawElement[]) {
     return JSON.stringify(
-      elements.map(element => ({ ...element, isSelected: false }))
+      elements.map(({ shape, ...element }) => ({
+        ...element,
+        isSelected: false
+      }))
     );
   }