Fix for Issue #12

- fixed errors reported by valgrind
Performance optimizations:
1. Triangle rasterization: replaced loops over all the grid cells of the AABB with looping over the area of the triangle itself (i.e. bounds are set per row).
2. Region expansion: instead of sweeping through all the cells at each expandRegion() and finding the right level - sweep once per few levels and divide among several stacks. Left over are appended.
Rasterization optimization: replaced clipping for each individual cell. the clipping line/plane between two adjacent cells is common so data from former cell can be reused if we keep track of the "remaining polygon".
Optimization of of the mesh detail construction: replaced the first of the 2 flood-fill algorithms (the one used to find the span corresponding to the center of the polygon) with a search of span at the center with the region matching the polygon.
Fixed item type in PolyRefArray from int to dtPolyRef
Merge branch 'master' of https://github.com/axelrodR/recastnavigation into axelrodR-master
Small formatting change to dividePoly()
gitignore meshes starting with underscore
Fix for detail mesh corner cases

- fix case where thing long poly center may not be on the region
bugfix: minor typo caused stack overrun when several vertices happen to fall exactly on cell boundaries
bugfix in RecastMeshDetail: in some rare dense cases the polygon simplification moves the polygon to a point where the center is very far from the region. For now we just fall back to the old code (flood-fill from the vertices)
changed remarks
Merge branch 'master' of https://github.com/memononen/recastnavigation
thrown away the fix I made to Recast/Source/RecastMeshDetail.cpp in favor of the upstream code.
Merge pull request #14 from axelrodR/master

Bugfix: A typo in dividePoly can cause stack overrun when several vertices happen to fall exactly on cell boundaries
Fix for U-turn path case
bugfix: divPoly could produce an 8th point after several calls because some of the old clipPoly code on which it was based
can add the same point twice consecutively (interpolation+adding the point).
Rather than raising the buffer I rewrote divPoly to avoid adding the same point twice.
Merge pull request #15 from axelrodR/master

bugfix: divPoly caused a stack overrun by producing an 8th point