Add Platform.SystemSupportDir for shared user data.

OpenRA.Game/Platform.cs

@@ -64,15 +64,19 @@ namespace OpenRA
 			}
 		}
 
+		/// <summary>
+		/// Directory containing user-specific support files (settings, maps, replays, game data, etc).
+		/// The directory will automatically be created if it does not exist when this is queried.
+		/// </summary>
 		public static string SupportDir { get { return supportDir.Value; } }
 		static Lazy<string> supportDir = Exts.Lazy(GetSupportDir);
 
 		static string GetSupportDir()
 		{
-			// Use a local directory in the game root if it exists
-			var supportDir = Path.Combine(GameDir, "Support");
-			if (Directory.Exists(supportDir))
-				return supportDir + Path.DirectorySeparatorChar;
+			// Use a local directory in the game root if it exists (shared with the system support dir)
+			var localSupportDir = Path.Combine(GameDir, "Support");
+			if (Directory.Exists(localSupportDir))
+				return localSupportDir + Path.DirectorySeparatorChar;
 
 			var dir = Environment.GetFolderPath(Environment.SpecialFolder.Personal);
 
@@ -95,6 +99,33 @@ namespace OpenRA
 			return dir + Path.DirectorySeparatorChar;
 		}
 
+		/// <summary>
+		/// Directory containing system-wide support files (mod metadata).
+		/// This directory is not guaranteed to exist or be writable.
+		/// Consumers are expected to check the validity of the returned value, and
+		/// fall back to the user support directory if necessary.
+		/// </summary>
+		public static string SystemSupportDir { get { return systemSupportDir.Value; } }
+		static Lazy<string> systemSupportDir = Exts.Lazy(GetSystemSupportDir);
+
+		static string GetSystemSupportDir()
+		{
+			// Use a local directory in the game root if it exists (shared with the system support dir)
+			var localSupportDir = Path.Combine(GameDir, "Support");
+			if (Directory.Exists(localSupportDir))
+				return localSupportDir + Path.DirectorySeparatorChar;
+
+			switch (CurrentPlatform)
+			{
+				case PlatformType.Windows:
+					return Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.CommonApplicationData), "OpenRA") + Path.DirectorySeparatorChar;
+				case PlatformType.OSX:
+					return "/Library/Application Support/OpenRA/";
+				default:
+					return "/var/games/openra/";
+			}
+		}
+
 		public static string GameDir
 		{
 			get