Imagery tiles requested when hidden behind other imagery

Imagery tiles are requested regardless of their visibility. If an imagery layer is below another opaque imagery layer its tiles will still get requested and rendered. Is it possible to detect this and not request those tiles?

Example:

```js
var viewer = new Cesium.Viewer('cesiumContainer');
viewer.imageryLayers.addImageryProvider(Cesium.createTileMapServiceImageryProvider({
url : Cesium.buildModuleUrl('firstlayer')
}));
viewer.imageryLayers.addImageryProvider(Cesium.createTileMapServiceImageryProvider({
url : Cesium.buildModuleUrl('secondlayer')
}));
```

Reproduce the behavior with the two-layer JavaScript example in the issue, then trace how imagery layers request and render tiles. Determine how opaque layers affect lower layers, and verify completion by confirming that fully hidden imagery tiles are neither requested nor rendered while visible imagery remains unchanged.

Written by the indexing model from the issue text.