Design Proposal: Add encoding field to vector source to allow for alternative tile formats.

Motivation

To support MapLibre Tiles, or other formats, MapLibre needs to know that a source is in that format.

Proposed Change

Add an encoding field to vector source, closely based on the same field from raster source.

The default is "mvt," indicating the existing behavior.

Examples

These are equivalent, both producing the same results as any previous style:

"source-vector-default": {
      "type": "vector",
      "url": "https://demotiles.maplibre.org/tiles/tiles.json"
    },
    "source-vector-mapbox": {
      "type": "vector",
      "url": "https://demotiles.maplibre.org/tiles/tiles.json",
      "encoding": "mvt"
    },

This would cause MapLibre to expect MLT-format tiles:

    "source-vector-maplibre": {
      "type": "vector",
      "url": "https://demotiles.maplibre.org/mlt-tiles/tiles.json",
      "encoding": "mlt"
    },

API Modifications

No API changes are necessary.

Migration Plan and Compatibility

Proposed changes are fully backward-compatible, so no migration is needed.

Rejected Alternatives

A new source type.

References

#1251