[Cesium3DTileStyleEngine] Missing support of access to values which are not in batch table, but present

It is currently not possible to access coordinates from a Cesium3DTileStyle.

An example would be:
```
tileset.style = new Cesium3DTileStyle({
"color" : {
"conditions" : [
["(${x} < ${offset} || ${x} > ${offset} + ${maxHeight})", "rgb(255, 255, 255)"],
[true, "rgb((${x} - ${offset})/${maxHeight}, 230, 230)"],
]
}
});
```

It would be also nice if you have access to BoundingBoxes and other properties of a tileset, so a support of following attributes would be nice:
* x/y/z
* bounding box of current tile
* bounding box of root tileset
* level in tree of current tile, starting from root tileset

Start with the Cesium3DTileStyleEngine and Cesium3DTileStyle entry points, then trace how style expressions currently access batch-table values. Clarify the scope for coordinates, current and root tile bounding boxes, and tree level; done means the agreed attributes can be used in the listed style conditions.

Written by the indexing model from the issue text.