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
Merge branch 'master' of https://github.com/memononen/recastnavigation