Add D2K random map generator

Adds a random map generator tailored for the Dune 2000 mod,
D2kMapGenerator.

This map generator has some limitations compared to the RA/CnC
map generator (ExperimentalMapGenerator):

- Symmetry is far less accurate for non-trivial rotations.
- No circular map shapes.
- Unplayable regions are not obstructed and no symmetry corrections
  are applied. This is partly due to mod limitations (e.g., no
  decorative actors), but I've also considered it less important.
- Somewhat higher map generation failure rates, due to more extreme
  play-space requirements.

In order to support the map generator, some additional features are
added to common map generation utilties:

- A "FromTemplates" MultiBrush collection shorthand.
- Path partitioning logic in Terraformer, use to split paths into
  multiple TilingPaths with different segment types.

mods/d2k/mod.yaml

@@ -53,6 +53,7 @@ Rules:
 	d2k|rules/starport.yaml
 	d2k|rules/husks.yaml
 	d2k|rules/arrakis.yaml
+	d2k|rules/map-generators.yaml
 
 Sequences:
 	d2k|sequences/aircraft.yaml