CollectionView cell background makes elevation shadow disappear

### Problem description

When creating a `CollectionView` cell with a background color and another view overlapping the `CollectionView`, the _elevation_ shadow of the ovelapping view is not shown.

### Expected behavior

The elevation shadow should remain visible on overlapping views.

### Environment

- Tabris.js version: 2.0.0
- Device: iPad Pro 9.5"
- OS: 10.3.3

### Code snippet

Removing the `background` from the `TextView` shows the elevation shadow as expected.

```javascript
const {CollectionView, TextView, Composite, ui} = require('tabris');

new Composite({
left: 0, right: 0, top: 0, height: 48,
background: 'white', elevation: 8
}).appendTo(ui.contentView);

new CollectionView({
left: 0, top: 'prev()', right: 0, bottom: 0,
itemCount: 10,
cellHeight: 48,
createCell: () => new TextView({text: 'item', background: '#eeeeee'}),
}).appendTo(ui.contentView);

```