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/element/newElement.ts

@@ -1,5 +1,6 @@
 import { randomSeed } from "../random";
 import nanoid from "nanoid";
+import { Drawable } from "roughjs/bin/core";
 
 export function newElement(
   type: string,
@@ -28,7 +29,8 @@ export function newElement(
     roughness,
     opacity,
     isSelected: false,
-    seed: randomSeed()
+    seed: randomSeed(),
+    shape: null as Drawable | Drawable[] | null
   };
   return element;
 }