Re-render when adding/removing primitives in requestRenderMode

In the `development/Polylines On Terrain` sandcastle example, if you initialize the viewer with...
```
var viewer = new Cesium.Viewer('cesiumContainer', {
terrainProvider: worldTerrain,
requestRenderMode: true,
});
```
The "Toggle instance show" button stops working after initial render.

In an app this prevents these polylines from initially appearing even if you `requestRender()` when adding the geometry.

Start with the development/Polylines On Terrain Sandcastle example and reproduce the failure after enabling requestRenderMode. Trace how adding or removing primitives and the "Toggle instance show" button interact with requestRender(). Done means the button works after initial render and newly added polylines appear when rendering is explicitly requested.

Written by the indexing model from the issue text.