View tiles in specification

Hi,

Is there any reason, why [`tiles`](https://github.com/mapbox/mbtiles-spec/blob/master/1.3/spec.md#tiles) is required as a table? It is commonly used as `view` over more tables, like `images` and `map`.

Even in your (MapBox) node-MBTiles utility, you are using `view`, not `table` type for the `tiles`: [node-mbtiles schema](https://github.com/mapbox/node-mbtiles/blob/master/lib/schema.sql#L54)

The [`metadata`](https://github.com/mapbox/mbtiles-spec/blob/master/1.3/spec.md#metadata) has correctly `table or view`.

Start with the Tiles section in 1.3/spec.md and compare its requirement with the linked node-MBTiles schema.sql and the Metadata wording. Determine whether tiles should permit a view as well as a table, then document the decision and its rationale. Done means the specification question is resolved in the relevant documentation.

Written by the indexing model from the issue text.