Optimize off mesh connections
See merge request OpenMW/openmw!969
(cherry picked from commit 9dcea247d2cd7d25d719fabc142cef5360233e2a)
3e98db8d Fix styleguide
7f65a2c4 Remove unused code
81e569c3 Move OffMeshConnectionsManager implementation into cpp
a8ba9a0e Cleanup unused tile positions from OffMeshConnectionsManager
ff1af5e8 Use only off mesh connections starting or ending in a given tile
1552e7e3 Add pathgrid edges as one direction off mesh connection
