Fix positioning during page flip for large books (#1078)

Large books with leaf edges had weird page offsets/shifts during the
page flip animation as a result of the previous anim fix (#1077).
This change ensures the page is positioned correctly throughout the
animation regardless of whether leaf edges are present.

src/BookReader/Mode2Up.js

@@ -847,7 +847,7 @@ export class Mode2Up {
       // Ensure the new left leaf is right-positioned before animating its width.
       // Otherwise, it animates in the wrong direction.
       this.pageContainers[newIndexL].$container.css({
-        right: `${newWidthL}px`,
+        right: `${$twoPageViewEl.prop('clientWidth') - gutter}px`,
         left: ''
       });
       this.pageContainers[newIndexL].$container.animate({width: `${newWidthL}px`}, speed, 'easeOutSine', () => {