Fix crash on startup with KSA r3646 (#57) (#58)

KSA r3646 added `Directory.SetCurrentDirectory(AppDomain.CurrentDomain.BaseDirectory)`
at the top of Program.Main() to fix a Linux startup crash. When launched via StarMap,
AppDomain.CurrentDomain.BaseDirectory resolves to StarMap's directory instead of the
game directory, causing the game to look for assets in the wrong location.

Fix by setting APP_CONTEXT_BASE_DIRECTORY to the game directory, which is the backing
store for AppDomain.CurrentDomain.BaseDirectory in .NET.

Fixes #55

Co-authored-by: Maximilian Neßlauer <maximiliannesslauer@gmail.com>

Author: KlaasWhite

StarMap.Loader/GameSurveyer.cs

@@ -40,6 +40,7 @@ public bool TryLoadCoreAndGame()
             }
 
             Directory.SetCurrentDirectory(gameDirectory);
+            AppContext.SetData("APP_CONTEXT_BASE_DIRECTORY", gameDirectory + Path.DirectorySeparatorChar);
 
             _core = core;
             core.Init();