Let getVisibleElements return a Set containing the visible element ids

Note how in `PDFPageViewBuffer.resize` we're manually iterating through the visible pages in order to build a Set of the visible page `id`s. By instead moving the building of this Set into the `getVisibleElements` helper function, as part of the existing parsing, this code becomes *ever so slightly* more efficient.

Furthermore, more direct access to the visible page `id`s also come in handy in other parts of the viewer as well.
In the `BaseViewer.isPageVisible` method we no longer need to loop through the visible pages, but can instead directly check if the pageNumber is visible.
In the `PDFRenderingQueue.getHighestPriority` method, when checking for "holes" in the page layout, we can also avoid some unnecessary look-ups this way.

web/ui_utils.js

@@ -473,6 +473,7 @@ function getVisibleElements({
   }
 
   const visible = [],
+    ids = new Set(),
     numViews = views.length;
   let firstVisibleElementInd = binarySearchFirstItem(
     views,
@@ -558,6 +559,7 @@ function getVisibleElements({
       percent,
       widthPercent: (fractionWidth * 100) | 0,
     });
+    ids.add(view.id);
   }
 
   const first = visible[0],
@@ -572,7 +574,7 @@ function getVisibleElements({
       return a.id - b.id; // ensure stability
     });
   }
-  return { first, last, views: visible };
+  return { first, last, views: visible, ids };
 }
 
 /**