Fix scroll bug in spotlight view

This was a confusion between indicies of the tile and the tile position:
the spotlight tile is the 0th TilePosition, ie. the tile with order
0, not the tile with index 0.

Also comment one method to hopefully make this slightly easier to
understand.

src/video-grid/VideoGrid.tsx

@@ -660,6 +660,8 @@ function getSubGridPositions(
   return newTilePositions;
 }
 
+// Sets the 'order' property on tiles based on the layout param and
+// other properties of the tiles, eg. 'focused' and 'presenter'
 function reorderTiles(tiles: Tile[], layout: Layout) {
   if (layout === "freedom" && tiles.length === 2) {
     // 1:1 layout
@@ -904,12 +906,12 @@ export function VideoGrid({
         return {
           x:
             tilePosition.x +
-            (layout === "spotlight" && tileIndex !== 0 && isMobile
+            (layout === "spotlight" && tile.order !== 0 && isMobile
               ? scrollPosition
               : 0),
           y:
             tilePosition.y +
-            (layout === "spotlight" && tileIndex !== 0 && !isMobile
+            (layout === "spotlight" && tile.order !== 0 && !isMobile
               ? scrollPosition
               : 0),
           width: tilePosition.width,