[fix] ImageBackground with <Text> children

Prevent Text from displaying behind the Image.

packages/react-native-web/src/exports/ImageBackground/index.js

@@ -63,7 +63,8 @@ class ImageBackground extends Component<*> {
               // This workaround should be removed after implementing proper support of
               // intrinsic content size of the <Image>.
               width: style.width,
-              height: style.height
+              height: style.height,
+              zIndex: -1
             },
             imageStyle
           ]}