Instanced model position in the styling language

[lilleyse]

It might be useful to introduce a built-in property ${POSITION} to the styling language so that we can get the position of an instanced model. Point clouds already have this property.

Some use cases are coloring by distance (like a fade effect?) or hiding instances in the tile that are further away.

Inspired by https://groups.google.com/forum/#!topic/cesium-dev/D-CROeGeRiY

[lilleyse]

Noted in #2 (comment)