Document maskLevel/fillzoom

[sfkeller]

I'm pretty sure that there are TileJSON files - as well as services and data - around which contain maskLevel.
The logic is described here: https://github.com/mapbox/tilelive-vector
Should'nt this be part of the spec.?

[mapsam]

Thanks for bringing this up! Looks like maskLevel is now referred to as fillzoom, we’ll revisit when we start researching and drafting up the next versions.

[mapsam]

Let's plan on having fillzoom documented in #36 since this can be a complicated concept and would be great to have a single point of reference for other users.

Right now the most this is documented is in tilelive-vector with:

maskLevel tiles: to avoid requiring many duplicate or empty vector tiles to be generated at high zoom levels, the backend source can specify a maskLevel. If a vector tile is not initially found at some z > maskLevel, Vector will issue an additional request to the backend using the parent tile of of the request at maskLevel. This allows a lower zoom level to "backfill" high zoom levels.

and

fillzoom: tile from the fillzoom of the backend (backfilled from a lower zoom)

I think we can do better, and provide more concrete examples for why you would use this property.

cc @springmeyer @GretaCB