Integrate the C++ MLT encoder from maplibre-tile-spec as a submodule.
Tiles are encoded by converting the existing mvt_tile to
mlt::Encoder::Layer and calling the encoder library directly.
New CLI flags:
--output-format=mlt Encode tiles as MLT instead of MVT
--pretessellate Pre-triangulate polygons (MLT only)
--no-mlt-feature-sort Disable within-tile Hilbert sorting (MLT only)
MLT tiles use .mlt extension in directory output and format=mlt in
mbtiles metadata. Compression and all existing flags work unchanged.
