Option to render Geometry Tiles as solid objects

Geometry Tiles are always rendered as classification primitives. We should provide the option to render them as solid shapes as well.

> In addition to rendering the geometry, the geometry can be used for classifying geometry of other 3D Tiles tilesets.

From [Geometry Tiles](https://github.com/AnalyticalGraphicsInc/3d-tiles/tree/3d-tiles-next/TileFormats/Geometry). Basically we only do the classification part currently.

Requested here: https://github.com/AnalyticalGraphicsInc/3d-tiles/issues/247#issuecomment-413001906

Start by reading the Geometry Tiles specification linked in the issue and tracing Cesium's current handling of Geometry Tiles as classification primitives. Identify the rendering entry point and existing classification tests, then define the option and verify that Geometry Tiles can also be rendered as solid shapes without losing classification behavior.

Written by the indexing model from the issue text.