feat: add cross-engine platform services

Runtime/SaveTypes.cs

@@ -63,7 +63,11 @@ namespace ShrinkDataSaver
     {
         public string SlotName = "New Save";
         public bool CaptureScreenshot = false;
+#if UNITY_5_3_OR_NEWER
         public UnityEngine.Texture2D Screenshot = null;
+#else
+        public byte[] Screenshot;
+#endif
         public bool Encrypt = false;
         public string EncryptionKey = null;
     }