Add support for shared depots (depotfromapp).

DepotDownloader/ContentDownloader.cs

@@ -208,6 +208,21 @@ namespace DepotDownloader
             if (depotChild == KeyValue.Invalid)
                 return INVALID_MANIFEST_ID;
 
+            if (depotChild["depotfromapp"] != KeyValue.Invalid)
+            {
+                uint otherAppId = (uint)depotChild["depotfromapp"].AsInteger();
+                if (otherAppId == appId)
+                {
+                    // This shouldn't ever happen, but ya never know with Valve. Don't infinite loop.
+                    Console.WriteLine("App {0}, Depot {1} has depotfromapp of {2}!",
+                        appId, depotId, otherAppId);
+                    return INVALID_MANIFEST_ID;
+                }
+
+                steam3.RequestAppInfo(otherAppId);
+                return GetSteam3DepotManifest(depotId, otherAppId, branch);
+            }
+
             var manifests = depotChild["manifests"];
             var manifests_encrypted = depotChild["encryptedmanifests"];