Added several methods in Heightfield for triangle rasterization.
RasterizeTriangle(ref Vector3, ref Vector3, ref Vector3, [AreaFlags]) is now the overload that does the actual rasterization, updated Triangle3 for this.
Marked SharpNav's internals as visible to SharpNavTests which, as a result, is now strong-name signed.
Moved Heightfield's private helper methods to HeightfieldHelper as internal static methods to make testing easier.