fix(ios): itemOverlap zIndex fix

Author: farfromrefug

src/collectionview/index.ios.ts

@@ -1242,6 +1242,9 @@ class UICollectionViewFlowLayoutImpl extends UICollectionViewFlowLayout {
                 const attributes = attributesArray.objectAtIndex(index);
                 if (attributes.representedElementCategory === UICollectionElementCategory.Cell) {
                     const row = attributes.indexPath.row;
+                    if (owner.itemOverlap) {
+                        attributes.zIndex = row;
+                    }
                     const itemOverlap = owner.itemOverlap(owner.getItemAtIndex(row), row);
                     const xPosition = attributes.center.x + Utils.layout.toDeviceIndependentPixels(Length.toDevicePixels(itemOverlap[1], 0) + Length.toDevicePixels(itemOverlap[2], 0)) * row;
                     const yPosition = attributes.center.y + Utils.layout.toDeviceIndependentPixels(Length.toDevicePixels(itemOverlap[0], 0) + Length.toDevicePixels(itemOverlap[2], 0)) * row;