tiles: skip layers without data when combining vector tiles

[cportele]

Part of addressing ldproxy/ldproxy#1722

A tileset that has no data in the area of a tile cannot contribute a layer to a combined tile, so its tile is now neither looked up in the cache nor generated. Before, a tile that was not in the cache was generated on the fly for every source tileset, which made combining a tile as expensive as generating all of its layers.

• TileEncoderMvt: transform the tile bounds once per combined tile and skip a layer when the bounds of its tileset do not intersect them; an unknown extent is treated as "may have data"
• TileEncoders: hand the tileset bounds and the CRS transformer factory to the encoders, which are instances now instead of a static map
• TileProviderFeatures: expose the bounds of a tileset from its metadata
• TileSeedingJobCreator: a job set whose tilesets are not covered by any seeded cache produces no jobs, so its total stayed unknown; the job set was neither finished nor removed from the queue: it was reported at 0% indefinitely and it suppressed every following seeding run of the tile provider; such a job set now has a total of zero and completes through the regular cleanup