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.

main.hpp

@@ -76,4 +76,11 @@ bool progress_time();
 
 #define MAX_ZOOM 24
 
+#define OUTPUT_MVT 0
+#define OUTPUT_MLT 1
+
+extern int output_format;
+extern bool mlt_sort_features;
+extern bool mlt_pretessellate;
+
 #endif