Vary the MVT extent so 8192 is only on max_zoom

[nvkelso]

Right now we use the MVT extend of 8192 for all zooms, but that's only really useful for overzooming the max_zoom tiles in the browser from say zoom 16 to zoom 22. The earlier zooms can be set to 4096.

[nvkelso]

Via tilezen/tilequeue#404