Add button when scrolled outside of visible area (#643)

With the infinite scroll behavior, it's easy to scroll super far away from where the content is and have a hard time getting back. This PR adds a button to refocus on the center of the scene when no elements are visible anymore.

src/scene/data.ts

@@ -46,7 +46,7 @@ export function serializeAsJSON(
   );
 }
 
-function calculateScrollCenter(
+export function calculateScrollCenter(
   elements: readonly ExcalidrawElement[],
 ): { scrollX: number; scrollY: number } {
   let [x1, y1, x2, y2] = getCommonBounds(elements);

src/scene/index.ts

@@ -17,6 +17,7 @@ export {
   importFromBackend,
   addToLoadedScenes,
   loadedScenes,
+  calculateScrollCenter,
 } from "./data";
 export {
   hasBackground,