Testing tile ordering based on tileid.z

- vertex position's z is modified based on tileid.z.
- For testing this approach, a uniform is set to modify the z of the vertex position to "1.0 + log(tileID.z)". This
helps in avoiding overlapping geometry drawing when drawingparent proxy tiles and corresponding child proxy tiles.
(Note: overlapping geometry issues does not occur when drawing child proxies for a visible parent tile).
- Question?? If we use this approach, does it make sense to store the vertex position to account for this z reorder, and
not using a uniform at shade time?
Merge branch 'cont-zoom' of https://github.com/tangram-map/tangram-es into cont-zoom