Signal fixes and updates — Steam Revolution: The Age of Rail Steam update #47417

Fixed bug where depots directly touch a block with platforms that load were not handled properly. This could lead to priority stealing, and non-determinism in multi-threading. This was a rare and subtle simulation problem, but required some rethinking of simulation internals. Added internal signal type start as part of the code refactor.Fix a bug where if there are multiple paths in a block where one of the paths is invalid and touching a vertex that has a valid path traveling in the opposite direction that it could sometimes incorrectly generate a path with the train reversing in an impossible way. This happened because I was just storing a shortest distance per segment, whereas I should have been storing a shortest distance per segment per direction. Fixed a bug where the number of signals counted for cost was incorrect because it counted implicit signals (terminal, start), which are n...