Added new method to partition heighfield

- added layer based heighfield partitioning
- the method is a bit slower than monotone partitioning, but does not
suffer from the long thin ploys
- the method partitions the heighfield into non-overlapping areas, but
does not try to resolve holes
- improved contour hole merging so that it can properly handle all
kinds of holes
- improved polygon triangulation to handle overlapping segments
- improved small and long polygon detail mesh generation
- updated samples to include all 3 partition methods and little
documentation to help to choose between them

Recast/Source/RecastLayers.cpp

@@ -38,7 +38,7 @@ struct rcLayerRegion
 	unsigned char layerId;		// Layer ID
 	unsigned char nlayers;		// Layer count
 	unsigned char nneis;		// Neighbour count
-	unsigned char base;			// Flag indicating if the region is hte base of merged regions.
+	unsigned char base;			// Flag indicating if the region is the base of merged regions.
 };