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.

.gitignore

@@ -43,6 +43,9 @@ tests/**/*.mbtiles
 tests/**/*.check
 tests/**/*.geobuf
 
+# MLT build directory
+mlt-build/
+
 # Vim
 *.swp
 
@@ -51,3 +54,4 @@ tests/**/*.geobuf
 
 # Nodejs
 node_modules
+tmp/