RFC: Support Vector Tiles, i.e. Z/X/Y filtering [#2690877]

Problem/Motivation

Vector tiles are a new alternative to clipping data sets that might offer a more efficient approach than bbox for certain applications. Presumably this would enable more efficient caching, which would help to facilitate efficient server side clustering. See:

Proposed resolution

Add 3 contextual filter handlers for z, x,and y parameters. This might be best achieved by a standard filter which has the ability to select it's 3 parameters from available contextual filters, allowing developers the option of stacking additional contextual filters in the URL path without breaking compatibility. Then, actual contextual filters could be of type "Global: dummy".