calculate off-mesh-connection height correctly
Fix rcMergePolyMeshes() ignoring portals of input meshes.

Copy the portals from input meshes on border to the output mesh.
Fix for detail mesh height data generation

- if the new region based height data generation fails, use the old
flood fill based method
Merge pull request #16 from qiqian/master

calculate off-mesh-connection height correctly
Merge pull request #18 from jackpoz/rcMergePolyMeshes_portals_fix

Fix rcMergePolyMeshes() ignoring portals of input meshes.
Merge remote-tracking branch 'upstream/master'