Add MLT (MapLibre Tile) output format

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.

mlt.hpp

@@ -0,0 +1,9 @@
+#ifndef MLT_HPP
+#define MLT_HPP
+
+#include <string>
+#include "mvt.hpp"
+
+std::string encode_as_mlt(const mvt_tile &tile, bool sort_features, bool pretessellate);
+
+#endif